you're going to need an equation that can find the acceleration (or deceleration) when given an initial as well as final velocity and time.
I suggest
Vf=Vo+at
that should give you acceleration
I believe you're over thinking this one. The problem clearly states to ignore any frictional force of air drag or frictional forces of the trailer. Therefore, thinking about the system as a whole, the net force would simply equal the mass of the system as a whole times the acceleration of the...