Need VB.net help please.....

Question

Hi guys Ive got a from with 4 textboxes on it, cost,selling,m/up,gp
when i enter cost and selling it works out the gp and m/up



Private Sub TextBox2_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Selling.TextChanged
        Try
            mup.Text = Format(Val((Selling.Text) - Val(Cost.Text)) / Val(Cost.Text) * 100, "0.00")
            gp.Text = Format(Val((Selling.Text) - Val(Cost.Text)) / Val(Selling.Text) * 100, "0.00")
        Catch ex As Exception

        End Try

    End Sub

this works but now i want the selling,gp and m/up to change when i change the gp and i cant seem to get that working
i've got the Fromula for that but i keep getting errors or it doesn't change


this is the second part


  Private Sub TextBox4_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles gp.TextChanged
        Try
            'selling =              cost *            (1 + m/up%)
            Selling.Text = Val(Cost.Text) * (1 + Val(mup.Text) / 100)


        Catch ex As Exception

        End Try
    End Sub

any help would really be appreciated

thanks