Need help adding two matrices on Visual Basic

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VB

Hi there, I'm doing a project for one of my classes and I'm stuck trying to add two matrices. I managed to make the matrix via keypress, but when I input details for the calculate function (cals_click), I cannot find a way to add two matrices (which in design would be under ListA and ListB boxes). How do I approach this?

Private Sub Input_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles Input.KeyPress
    Dim x1, x2, x3 As String
    Dim x23 As String
    Dim pos1, pos2 As Integer

    If Asc(e.KeyChar) = 13 Then
        pos1 = InStr(Input.Text, ";")
        x1 = Mid(Input.Text, 1, pos1 - 1)
        x23 = Mid(Input.Text, pos1 + 2)
        pos2 = InStr(x23, ";")
        x2 = Mid(x23, 1, pos2 - 1)
        x3 = Mid(x23, pos2 + 2)

        If MA.Checked Then
            ListA.Items.Add(x1)
            ListA.Items.Add(x2)
            ListA.Items.Add(x3)

        End If
        If MB.Checked Then
            ListB.Items.Add(x1)
            ListB.Items.Add(x2)
            ListB.Items.Add(x3)
        End If
        If MC.Checked Then
            ListC.Items.Add(x1)
            ListC.Items.Add(x2)
            ListC.Items.Add(x3)
        End If
        Input.Text = ""

    End If
End Sub

Private Sub midinstr(ByVal x123 As String, ByRef x1 As Single, ByRef x2 As Single, ByRef x3 As Single) 'byref = input, byval = output
    Dim x23 As String 'seperator
    Dim pos1, pos2 As Integer 'seperator

    pos1 = InStr(x123, ",")
    x1 = Val(Mid(x123, 1, pos1 - 1))
    x23 = Mid(x123, pos1 + 1)
    pos2 = InStr(x23, ",")
    x2 = Val(Mid(x23, 1, pos2 - 1))
    x3 = Val(Mid(x23, pos2 + 1))



End Sub


Private Sub Cals_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Cals.Click
    Dim A123, a2123, a3123 As String
    Dim a1, a2, a3 As Single
    Dim a21, a22, a23 As Single
    Dim a31, a32, a33 As Single
    Dim r As Double

    If DA.Checked Then
        A123 = ListA.Items(0)
        midinstr(A123, a1, a2, a3) 'from private sub below'
        a2123 = ListA.Items(1)
        midinstr(a2123, a21, a22, a23)
        a3123 = ListA.Items(2)
        midinstr(a3123, a31, a32, a33)

        r = a1 * (a22 * a33 - a23 * a32) - a2 * (a21 * a33 - a23 * a31) + a3 * (a21 * a32 - a22 * a31)

    End If

    If DB.Checked Then
        A123 = ListB.Items(0)
        midinstr(A123, a1, a2, a3) 'from private sub below'
        a2123 = ListB.Items(1)
        midinstr(a2123, a21, a22, a23)
        a3123 = ListB.Items(2)
        midinstr(a3123, a31, a32, a33)

        r = a1 * (a22 * a33 - a23 * a32) - a2 * (a21 * a33 - a23 * a31) + a3 * (a21 * a32 - a22 * a31)
    End If

    R_Val.Text = Str(r)

    If M1.Checked Then
        ListD.Items.Add()



    End If

End Sub