Particle 1 with mass $M$ has $x$ position defined by the function $x_{1}(t) = 2t (t + 3)$ where units are in meters, time is in seconds, and are abbreviated in the function. Particle 2 has mass $2M$ and $x$ position defined by $x_{2} (t) = (t-1)(t-2) / 2$.

Considering the system of these two particles, what is the **velocity** of the center of mass of the system at $t = 2$?