While rearranging a dorm room, a student does 300 J of work in moving a desk 2.0 m. What was the magnitude of the applied horizontal force?

Work is the amount of force applied times the displacement (or distance) of the object. The equation is
W = F(D) Force is calculated in Newtons and D is calculated in meters giving you a Newton-Meter which is commonly referred to as a Joule. To figure out your equation you must do some simple algebra to get your answer to the correct side of the equation. You are trying to get F alone, since you know how much work was done and the distance that means you need to get the force by itself. Your equation would then look like
W = F/D which transfers to W/D = F. Plug in your values and you have: 300J/2.0m = F. Working that problem gives you F = 150N

A train increases its velocity from 5miles/hr to 55miles/hr in 20sec. What is its acceleration?