Drop Down List value to Hide Rows

Try this in the worksheet module for sheet "RECYCLED CONTENT CALCULATOR"

Private Sub Worksheet_Change(ByVal Target As Range)
    If Target.Address = "$A$4" Then
        Range("A10:A19").EntireRow.Hidden = False
        If Target = "Double Wall" Then
            Range("A17:A19").EntireRow.Hidden = True
        ElseIf Target = "Single Wall" Then
            Range("A10:A17").EntireRow.Hidden = True
        End If
    End If
End Sub

Demoworkbook attached

Hope this helps