When you let u=\sin(x), you get:
\frac{du}{dx}=\cos(x) \implies du = \cos(x) dx
Your mistake is then substituting 'dx' for 'du'. (i.e. fifth line)
Moreover, you wrote \int u\,dx where it should technically be \int u\,du because you "substituted" 'du' in.
Try letting u=\cos(x).