$($a$)[2$ points$]$ Find a parametric equation for the line through the point $(1,5,2)$ and in the direction of the vector $2$vec$(i)+3$vec$(j)-$vec$(k).$

$($b$)[3$ points$]$ By minimizing the square of the distance from a point on the line to the origin, find the exact point on the line which is closest to the origin.