A 2.0 kg toy locomotive is pulling a 1.0 kg caboose. The frictional force of the track on the caboose is 0.50 N backward along the track. Of the train is accelerating foward at 3.0 m/s^2, what is the magnitude of the force exerted by the locomotive on the caboose?

So I'm guessing that I use the equation: F = ma, but I'm not sure how to get started. The answer in the back of the book is: 3.5 N. Thanks for your input~David.W