What to do if this error pops up?

Question

Visual Basic

The error pops up all the time:

Run-time error '5':
Invalid procedure call or argument

Here is the code

Function y(x)
y = 8.4 + (((x + 2.4) * Log(Abs(x))) / ((2 * x + 3) * (x + 8)))
End Function
Private Sub CommandButton2_Click()
Dim i As Double, step As Double, BoxCount As Double
i = 10
step = 0.1 * i
BoxCount = 1
    For x = 0 To i Step step
        UserForm1.Controls("TextBox" & BoxCount).text = x
        BoxCount = BoxCount + 1
        UserForm1.Controls("TextBox" & BoxCount).text = y(x)
        BoxCount = BoxCount + 1
    Next x
End Sub

By clicking on the Debug button, I get highlighted

y = 8.4 + (((x + 2.4) * Log(Abs(x))) / ((2 * x + 3) * (x + 8)))

Answer 1

variables y and x are not declared