integration by substitution

integral between -infinity and +infinity of



e^-iwt / (a - iw) dw





u = a - iw



du/dw = -i



dw = du / -i



e^-iwt = e^(u+a)t / -iu du




its a homework question, so just a pointer as to which method to use would be appreciated, it says substitution, but ive tried the above and doesnt get me anywhere, maybe im not choosing the correct substitution?



thanks for any help