$(15$ pts$)$ Consider the system shown in the following figure. The rope and pulley have negligible mass, and the pulley is frictionless. The coefficient of kinetic friction between the $8\mathrm{.}00-kg$ block and the tabletop is ${}_k=0\mathrm{.}250.$ The blocks are released from rest. Calculate the speed of the $6\mathrm{.}00-kg$ block after it has descended $0\mathrm{.}80$ m $.($This problem can be solved either by force laws or by energy conservation.$)$