Answer
(a) See the graph.
(b) $x^2+y^2=4,~~x\geq0$
Work Step by Step
(a)
If $t$ goes from $0$ to $\pi$, then $x$ goes from $0$ to $2$ and backs to $0$. At the same time $y$ goes from $2$ to $-2$
(b)
$x=2~sin~t$
$x^2=4~sin^2t$
$y=2~cos~t$
$y^2=4~cos^2t$
$x^2+y^2=4~sin^2t+4~cos^2t=4(sin^2t+cos^2t)$
$x^2+y^2=4,~~x\geq0$
