EXAMPLE $2$: A $210-$gram cart moves right at a speed of $5\mathrm{.}00$ m$/$s and collides with an unknown mass car moving left at a speed of $4\mathrm{.}00$ m$/$s$.$ They stick together and move toward the left at a speed of $0\mathrm{.}576$ m$/$s$.$ What is the unknown mass? $(342$ grams$)$ What is the momentum before the collision? $(-0\mathrm{.}318$ kg m$/$s$)$ What is the $\%$ change in the