Private Sub Worksheet_Change(ByVal Target As Range)
    If Target.Address = "$D$8" Or Target.Address = "$D$7" Or Target.Address = "$D$9" Then
        Call Module1.AutoGoalSeek
    End If
End Sub

http://www.excelforum.com/excel-programming/629622-run-macro-everytime-cell-content-changes.html

The easy way to set this up is to go to the macro window by pressing alt+F11.
Select the sheet from left panel. there will be two drop down. from the left drop down select worksheet and from right drop down list select the event 'change'.

Excel: Run Macro on Cell Change

excel

Visual Basic