$(15$ pts$)$ A loaded $425$ kg toboggan is traveling on smooth horizontal snow at $5\mathrm{.}4$m $/$ s when it suddenly comes to a rough region. The region is $8\mathrm{.}00$ m long and reduces the toboggan's speed by $1\mathrm{.}50$ m$/$s$.($ a$)$ What average friction force did the region exert on the toboggan? $($b$)$ What is the coefficient of kinetic friction between the toboggan and surface?