Visual Basic 2015, mấy bác giúp với

https://4share.vn/f/7d48494a444f4544

Public Class Form1

    Private Sub Insert_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Insert.Click
        If (IsNumeric(TVal.Text)) Then
            TListBox.Items.Add(TVal.Text)
            TVal.Text = ""
        End If
    End Sub


    Private Sub Calculate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Calculate.Click
        If (TListBox.Items.Count > 0) Then
            YListBox.Items.Clear()
            For i As Integer = 0 To TListBox.Items.Count - 1
                Dim t As Double = Convert.ToDouble(TListBox.Items(i).ToString())
                Dim y As Double


                If (t >= 0 And t < 5) Then
                    y = 1.5 * Math.Exp(-1.2 * t) + Math.Sqrt(t) + Math.Exp(-2.0 * t)
                    YListBox.Items.Add(y.ToString())
                End If
                If (t >= 5) Then
                    y = 2.5 * Math.Cos(t) + 1.2 * Math.Sin(t) + Math.Log10(t)
                    YListBox.Items.Add(y.ToString())
                End If
                If (t < 0) Then
                    y = 0
                    YListBox.Items.Add(y.ToString())
                End If
            Next
        End If
    End Sub
End Class