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Need help wit data entry validation: IsValidISBN()

I've been on this code for days now but I can't understand it. I'm not a programmer but I need the code for our school project. My current code is:

Private Function IsValidISBN(ByVal ISBN As String) As Boolean
Dim i, sum, cd As Integer
Dim checkDigit As Char
For i = 0 To ISBN.Length - 2
sum = sum + Val(ISBN.Chars(i)) * (10 - i)
Next
cd = (11 - sum Mod 11) Mod 11
If cd = 10 Then checkDigit = "X" Else checkDigit = cd.ToString()
If checkDigit = ISBN.Chars(i) Then Return True Else Return False
End Function

It works for an ISBN-10 number but I need the code to work for an ISBN-13 number, particularly these 2; 97803006406157, 9783161484100
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