Проблемка с решением задачки: три вектора

Option Strict On
Imports System.Math
Public Class Form1
    'Функция,вычисляющая выражение
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
 
    End Sub
    Function z(ByVal x1 As Double, x2 As Double, ByVal y1 As Double, y2 As Double) As Double
        x1 = CDbl(TextBox1.Text)
        y1 = CDbl(TextBox2.Text)
        x2 = CDbl(TextBox3.Text)
        y2 = CDbl(TextBox4.Text)
        z = Atan(y1 / x1) - Atan(y2 / x2)
        Return z 'Возвращение результата
    End Function
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim a As Double
        Dim x1, x2 As Double
        Dim y1, y2 As Double
        a = Acos((x1 * x2 + y1 * y2) / (Sqrt(x1 ^ 2 * y1 ^ 2)) * (Sqrt(x2 ^ 2 * y2 ^ 2)))
        TextBox5.Text = CStr(a)
        TextBox6.Text = CStr(z(x1, x2, y1, y2))
    End Sub
    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        'Действие при нажатии кнопки завершить 
        End
        'Закрытие окна программы 
    End Sub
End Class