probably loads of ways of going about this one.
try putting the below in the code window behind the actual worksheet
you are using (rather than in a normal module).
you should be able to adapt this to your needs

all the best
J

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$A$1" Then
If IsNumeric(Target) Then
Range("B1") = Target.Value + 2
End If
End If
End Sub