Public Class Form1

    Private Sub btnCaclulate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCaclulate.Click
        Me.lblArea.Text = Val(Me.tbLength.Text) * Val(Me.tbWidth.Text)
        Me.lblPerimeter.Text = 2 * Val(Me.tbLength.Text) + 2 * Val(Me.tbWidth.Text)
    End Sub

    Private Sub tbLength_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles tbLength.TextChanged
        Me.lblArea.Text = ""
        Me.lblPerimeter.Text = ""
    End Sub

    Private Sub tbWidth_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles tbWidth.TextChanged
        Me.lblArea.Text = ""
        Me.lblPerimeter.Text = ""
    End Sub
End Class