$14\mathrm{.}4$ Car A weighing $40001!?$ and car B weighing $3700$ are at rest on a $221$ton flatcar which is also at rest. Cars A and B then accelerate and quickly reach constant speeds relative to the flatcar of $7$ f$!/.$s and $3\mathrm{.}5$ f$!/.$s$,$ respectively, before decelerating to a stop at the opposite end of the flatcar. Neglecting friction and rolling resistance, determine the velocity of the flatcar when the cars are moving at constant speeds.