Hey sapsapz.

I think what you have to do is show that if the final transform has no complex part that the two are equal. If you have a complex variable z = x + iy, then y = 0 implies equality for your problem.

Recall that -e^(-iwx) = -[cos(-wx) + isin(-wx)] = -cos(-wx) + 0 (if no imaginary component) = -cos(wx) since cos(x) = cos(-x).