Hi majdi. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif
The line with ∫ T.sin3(t-T).dT is where you need to perform integration by parts, in order to get the next line.
How did you go from that line to the next? (BTW, that is the right answer after you evaluate that integral).
NB: I have forgotten convolution details, so I'm making no comment on your post before the integral by parts line.