Need help sum of textbox value

Dear Guru,
please help me ...
I have 3 textbox contains numbers (Integer)
and 2 textbox contains (+ or - or * or /)
I want to sum ​​3 texbox like
D = (A + B) - C
i use:
AB.Text = Val (A.Text) + Text5.Text + Val (B.Text) so
D.Text = Val (AB.Text) + Text6.Text + Val (C.Text)
but result did not match
there is a minus but the results do not fit

text5.text and text6.text containing (+ or - or * or /)

please someone tell me
thank you