A 250-kg snowmobile is traveling north at 12 m/s. Suppose that 15 s later it is going south at 18 m/s what is the acceleration

From what I think, we first find the relative speed, which is the difference in speeds from both directions, being 18-12 = 6 m/s. then acceleration is given by speed/time or 6/15=2/5 or 0.4 m/s^2. / b. Momentum is given by mass*velocity and that could be 250*3 = 750Kgms^-1