X/L = AuU* ((1-e)2 (1+30(1-f)3 ) / d2 )
It's really hard to tell what are the independent variables and the independent one
Assuming that
1. X/L is a single number (just written wrong for VBA)
2. AuU is a single number and not Au times U
3. (1-e)2 means (1-e) squared
4. (1-f)3 means (1-f) cubed
5. d2 means d squared
then a user defined function (UDF) in the workbook would be one way (below). The other would just be a long worksheet formula
Option Explicit
Function XoverL(AuU As Double, e As Double, f As Double, d As Double) As Double
XoverL = AuU * ((1 - e) ^ 2 * (1 + 30 * (1 - f) ^ 3) / d ^ 2)
End Function
Sub test()
MsgBox XoverL(100, 0.5, 12, 9)
End Sub
It's be a lot easier of offer suggestions if you attach a sample workbook with some dummy (but realistic) data (see my sig)