How do you simplify cosx(2sinx + cosx)-sin^2x?

You can start by multiplying $\cos \left(x\right)$ to get:
$2 \sin \left(x\right) \cos \left(x\right) + {\cos}^{2} \left(x\right) - {\sin}^{2} \left(x\right) =$ or:
$= \left[2 \sin \left(x\right) \cos \left(x\right)\right] + \left[{\cos}^{2} \left(x\right) - {\sin}^{2} \left(x\right)\right] =$
$= \sin \left(2 x\right) + \cos \left(2 x\right)$