In Excel, custom name a few cells. aval, bval, cval, inthigh, intlow, nval. This will stand for a, b, c, integral high value, integral low value, and integer n value. Now, in VB, do the following:

Dim a, b, c, ihigh, ilow, simp1, fa, fb, simp2, adb, simpson, As Double

Dim n as Integer

' Validate user input

' Inputs

a = Range("aval").Value

b = Range("bval").Value

c = Range("cval").Value

ihigh = Range("inthigh").Value

ilow = Range("intlow").Value

n = Range("nval").Value

If Len(a) = 0 Or Len(b) = 0 Or Len(c) = 0 Or Len(ihigh) = 0 Or Len(ilow) = 0 Or Len(n) = 0 Then

' Alert user that entries have not been filled out

MsgBox "You need to enter input values before continuing.", vbExclamation, "Warning Message"

Exit Sub

End If

simp1 = (b-a)/6

fa = (a * aval^2) + (b * aval) + c

adb = (a + b)/2

simp2 = 4 * ((a * adb^2) + (b * adb) + c)

fb = (a * bval^2) + (b * bval) + c

' Calculate Simpson value

simpson = simp1*(fa + simp2 + fb)

I got that formula from here:

http://en.wikipedia.org/wiki/Simpson's_Rule

You'll need to output the simpson value somewhere on the spreadsheet or a message box. Let me know if you need help with that. I put this code together quickly this morning. i'm not sure how much VB/Excel knowledge you have, so if you do have questions, by all means, ask.

I assume you are doing this in VBA to hide it from others, but it's very easy to do in Excel. I put in a warning message to validate user input, and made it an Exclamation warning to catch the user's eye if they make a mistake on entry points.