I need help super bad

Use the basic force equation and it's simple algebra from there.

F = ma

The sum of all your forces equals the mass times acceleration. Your force is 12 N and your acceleration is 3 m/s^2

actually fadedmaster is right follow the force equation... make sure u make a free body diagram of all the forces acting on the wagon and then sum them up in respective axis... (sum all the forces acting in y axis and so in x axis and so on)...