Answer
(a) $ -sin\theta$
(b) $ tan(x)$
Work Step by Step
Use Addition and Subtraction Formula, we have:
(a) $cos(270^\circ-\theta)=cos(270^\circ)cos(\theta)+sin(270^\circ)sin(\theta)=-sin\theta$
(b) $tan(\pi+x)=\frac{tan(\pi)+tan(x)}{1-tan(\pi)tan(x)}=tan(x)$