listbox sum

I'm a beginner that just can't get it through his dumb head what must be a very simple task - sum contents of a listbox.

Private Sub CommandButton2_Click()
Dim a As Single
For x = 0 To ListBox1.ListCount - 1
a = ListBox1.List(x)
b = a + x
Next
TextBox2.Text = b
End Sub

Any help would be very much appreciated.

change
b = a + x
to
b = a + b

Thanks for your help.