Rules of differentiation to find the derivative of the function.
pie/2 sin x - cos x
Not sure what to use here.
You're thinking that:
$\displaystyle \frac d {dx}\left({\frac {\pi} 2 \sin x - \cos x}\right) = \frac {\pi} 2 \cos x + \sin x$?
Well, you're right.
Diff of sine is cos, diff of cos is -sine, diff of a constant times a function is the constant times the diff of the function. Good job.