I have added a small example that shows a solution.
B4X:
Sub Globals
'Declare the global variables here.
c=0
End Sub
Sub App_Start
Form1.Show
End Sub
Sub Multiply(a,b)
Return a*b
End Sub
Sub Button1_Click
c=Multiply(TextBox1.Text,TextBox2.Text)
Label1.Text=c
End Sub
You can of course use other intermediate variables for the values in the textboxes.
Hm... there must be a typo or something like that in your personal code because the sample of klaus works fine. Take his attachment and try to compile it, it will work. So I'm sure you implemented his solution manually into your own code.
Error CS1525 occurs when the compiler detects an invalid character in an expression, so have a close look to you code again.