A particle starts at the point $P = \left(-3,1,-3\right)$ when $t = 0$ and moves along a straight line toward $Q = \left(0,6,-3\right)$ at a speed of $4$ cm/sec. Let x, y, and z be measured in cm, and t in seconds. Find a parametric vector equation for the position of the object.

$\vec{r}(t) =$