Friction force with crate on truck bed

A truck is traveling down a 15º incline. The truck then begind to break (decelerate) at a rate of 4 m/s. A crate sits on top of the truck bed..Determine the coefficient of friction to keep the crate from sliding on the truck bed. If the truck were to accelerate up a 15º incline at 4m/s^2 would the required coefficient of friction be the same?....

Im usually good with these problems but I feel like my prof. may have left out a mass of the crate...Is there a way to solve this problem without a mass? and if I need a mass what equation would I use to start this problem

Thanks everyone :)(Bow)