Answer
(a) $y=\frac{1}{2}x+2$, line, see figure.
(b) $x^2+y^2=36$, circle, see figure.
Work Step by Step
(a) Use the relation $x=r\ cos\theta, y=r\ sin\theta$, rewrite the equation as $2r\ sin\theta- r\ cos\theta =4$ which gives $2y-x=4$ or $y=\frac{1}{2}x+2$ which represents a line as shown in the figure.
(b) Use the relation $r^2=x^2+y^2$, we have $r^2=36$ and $x^2+y^2=36$ which represents a circle centered at $(0,0)$ with radius $6$ as shown in the figure.