Как исправить программу "ОДЗ функции"?

Public Class Form1
Dim x, a, b, st As Double
Dim w, h, mx, my, xc, yc As Integer
Dim g As Graphics
 
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
PictureBox1.BackColor = Color.White 'Цвет фона
Oval.Parent = PictureBox1
'параметры холста
w = PictureBox1.Width : h = PictureBox1.Height
Dim bmp = New Bitmap(w, h)
g = Graphics.FromImage(bmp)
 
'Расположение системы координат в окне
Dim xb, xe, yb, ye As Double
xb = -12 : xe = 12 : yb = 10 : ye = -10 'границы системы координат по x,y
Dim xlen, ylen As Integer
xlen = Math.Abs(Int(xb)) + Math.Abs(Int(xe)) : ylen = Math.Abs(Int(yb)) + Math.Abs(Int(ye)) 'длина между границами по x,y
mx = Math.Min(w / xlen, h / ylen) : my = mx 'масштаб по x,y(сколько пикселей в одной единице)
 
'границы в пикселях
Dim xbp, xep, ybp, yep As Integer
xbp = (w - mx * xlen) / 2 : xep = (w + mx * xlen) / 2
ybp = (h - my * ylen) / 2 : yep = (h + my * ylen) / 2
xc = xbp + mx * Math.Abs(Int(xb)) : yc = ybp + my * Math.Abs(Int(yb)) 'координаты центра в пикселях
 
'оси
Draw()
drawGraph(New Pen(Color.Red), -12.5, 12.5, 0.05) ' нарисовать график
 
PictureBox1.Image = bmp
 
a = -12.5 : b = 12.5 : st = TrackBar1.Value / 30
x = a
Timer1.Enabled = True
End Sub
 
 
Private Sub Draw()
Dim p = New Pen(Color.Blue)
g.DrawLine(p, New Point(0, yc), New Point(w, yc))
g.DrawLine(p, New Point(xc, 0), New Point(xc, h))
 
'x
Dim i As Integer
i = xc - mx
Do While i > 0
g.DrawLine(p, New Point(i, yc - 2), New Point(i, yc + 2))
i -= mx
Loop
 
i = xc + mx
Do While i < w
g.DrawLine(p, New Point(i, yc - 2), New Point(i, yc + 2))
i += mx
Loop
 
'y
i = yc - my
Do While i > 0
g.DrawLine(p, New Point(xc - 2, i), New Point(xc + 2, i))
i -= my
Loop
 
i = yc + my
Do While i < h
g.DrawLine(p, New Point(xc - 2, i), New Point(xc + 2, i))
i += my
Loop
 
'подписываем оси
g.DrawString("Y", New Font("Arial", 16), Brushes.Blue, New PointF(xc + 10, 10))
g.DrawString("X", New Font("Arial", 16), Brushes.Blue, New PointF(w - 20, yc - 40))
 
'Стрелки на осях
'x
g.DrawLine(p, New Point(w - 10, yc + 5), New Point(w, yc))
g.DrawLine(p, New Point(w - 10, yc - 5), New Point(w, yc))
'y
g.DrawLine(p, New Point(xc, 0), New Point(xc + 5, 10))
g.DrawLine(p, New Point(xc, 0), New Point(xc - 5, 10))
End Sub 'нарисовать систему координат
 
Function f(ByVal x As Double) As Double
f = Math.Sqrt(4 * x + 7) - 3 * Math.Cos(x)
End Function 'задаём функцию графика
 
Private Sub drawGraph(ByVal p As Pen, ByVal a As Double, ByVal b As Double, ByVal st As Double)
For x As Double = a To b Step st
XMLВыделить код
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  g.DrawLine(p, New Point(xc + Int(mx * x), yc - Int(my * f(x))), New Point(xc + Int(mx * (x + st)), yc - Int(my * f(x + st))))
Next
End Sub 'рисуем график
 
Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
If (x > b) Then x = a
Oval.Left = (xc + Int(mx * (x + st))) - Oval.Width / 2
Oval.Top = (yc - Int(my * f(x + st))) - Oval.Height / 2
x += st
End Sub 'передвигаем шарик

drawGraph(New Pen(Color.Red), 6, 12.5, 0.05)

Function f(ByVal x As Double) As Double
        If 4 * x + 7 = 0 OrElse 4 * x + 7 > 0 Then
            f = Math.Sqrt(4 * x + 7) - 3 * Math.Cos(x)
        Else
            Return 0
        End If
 
    End Function 'задаём функцию графика
 
    Private Sub drawGraph(ByVal p As Pen, ByVal a As Double, ByVal b As Double, ByVal st As Double)
        For x As Double = a To b Step st
            If 4 * x + 7 = 0 OrElse 4 * x + 7 > 0 Then
                p = New Pen(Color.Green)
            Else
                p = New Pen(Color.Red)
            End If
            g.DrawLine(p, New Point(xc + Int(mx * x), yc - Int(my * f(x))), New Point(xc + Int(mx * (x + st)), yc - Int(my * f(x + st))))
        Next
    End Sub 'рисуем график

  If 4 * x + 7 = 0 OrElse 4 * x + 7 > 0 Then