$($Section $12\mathrm{.}1,17\mathrm{.}2)$ Find parametric equations to describe the following curves.

$($a$)$ a line segment from $P(-2,1,0)$ to $Q(5,2,-2)$

$($b$)$ the right half of circle $x^2+y^2=9$ from $(0,-3)$ to $(0,3)$