Почему не роботает.?

Sub.Prl3
z = 0.2
x = -3
y = -2
Max = a(x)
Min = b(y)
DO WHILE (x + 0.13 <= 4)
    x = x + 0.13
    IF a(x) > Max THEN Max = a(x)
LOOP
DO WHILE (y + 0.17 <= 2)
    y = y + 0.17
    IF b(y) < Min THEN Min = b(y)
LOOP
msgBox (&Max + Min&)
FUNCTION a (x)
a = ((1 + x) * (x + z) * (x ^ 2 + 4)) / (EXP(-x - z) + (1 / (x ^ 2 + 4)))
END FUNCTION
FUNCTION b (y)
b = (y + COS(y - 2)) / (y ^ 4 / (2 + (SIN(z)) ^ 2))
END FUNCTION
END SUB

Sub Prl3 'Sub.Prl3
z = 0.2
x = -3
y = -2
Max = a(x)
Min = b(y)
DO WHILE (x + 0.13 <= 4)
    x = x + 0.13
    IF a(x) > Max THEN Max = a(x)
LOOP
DO WHILE (y + 0.17 <= 2)
    y = y + 0.17
    IF b(y) < Min THEN Min = b(y)
LOOP
msgBox (Max + Min) 'msgBox (&Max + Min&)
END SUB
 
FUNCTION a (x)
a = ((1 + x) * (x + z) * (x ^ 2 + 4)) / (EXP(-x - z) + (1 / (x ^ 2 + 4)))
END FUNCTION
 
FUNCTION b (y)
b = (y + COS(y - 2)) / (y ^ 4 / (2 + (SIN(z)) ^ 2))
END FUNCTION
'END SUB

Dim z
Sub Prl3()
 z = 0.2
 x = -3
 y = -2
 Max = a(x)
 Min = b(y)
 Do While (x + 0.13 <= 4)
 x = x + 0.13
 If a(x) > Max Then Max = a(x)
 Loop
 Do While (y + 0.17 <= 2)
 y = y + 0.17
 If b(y) < Min Then Min = b(y)
 Loop
 MsgBox (Max + Min)
  'msgBox (Max & "   " & Min)
 End Sub
 Function a(x)
 a = ((1 + x) * (x + z) * (x ^ 2 + 4)) / (Exp(-x - z) + (1 / (x ^ 2 + 4)))
 End Function
 Function b(y)
 b = (y + Cos(y - 2)) / (y ^ 4 / (2 + (Sin(z)) ^ 2))
 End Function
 
Private Sub Command1_Click()
Prl3
End Sub