Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As _
  System.EventArgs) Handles MyBase.Load
        ComboBox1.Items.Add("EGC-50-xxxx-TB-KF-GK")
        ComboBox1.Items.Add("EGC-70-xxxx-TB-KF-GK")
End Sub

Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As Object, ByVal e As _
  System.EventArgs) Handles ComboBox1.SelectedIndexChanged
        If ComboBox1.Items.Contains("EGC-50-xxxx-TB-KF-GK") Then TextBox3.Text _
        = 155
        If ComboBox1.Items.Contains("EGC-70-xxxx-TB-KF-GK") Then TextBox3.Text _
        = 246
End Sub