Мой VB.Net немного заржавел, но что-то вроде этого должно сделать это:
В событии SelectedIndexChanged укажите это:
'Make sure that we have a selected item before continuing
If listBox1.SelectedIndex = -1 Then
Return
End If
'Get the text of the selected item
Dim selectedtext As String = listBox1.Items(listBox1.SelectedIndex).ToString()
'Split the item by the + and the = into an array of strings
Dim parts As String() = selectedtext.Split("+"C, "="C)
'If we have more than 2 parts
If parts.Length > 2 Then
'Define a variable for each part
Dim part1 As String = parts(0).Trim()
Dim part2 As String = parts(1).Trim()
'Make text boxes set to part1 and part2. part1 = 1, part2 = 1
End If