Вычислить выражение - VB

Private Function min(x2 As Single) As Single
Dim x1 As Single
x1 = x2
If x2 < x1 Then x1 = x2
min = x1
End Function
Private Function max(x3 As Single) As Single
Dim x4 As Single
x4 = x3
If x3 > x4 Then x4 = x3
min = d
End Function
Private Sub CmdRun_Click()
Dim a As Single, b As Single, с As Single, z As Single
a = txta.Text
b = txtb.Text
с = txtc.Text
z = (max(a,b,c) - max^2(a+b,a*c,a*b))/(min(a*b,a*c,b*c)-1)
txtz.Text = CStr(z)
End Sub