The speed of a wagon increases from 2.5 meters per second to 9.0 meters per second in 3.0 seconds as it accelerates uniformly down a hill. what is the magnitude of the acceleration of the wagon during this 3.0 second time interval?

9 - 2.5 = 6.5 m/s
I am sure this is correct

GPE = mgh (for objects close to earth where g can be considered to be constant) Where m is the mass, g is the acceleration due to gravity (which the question has told us to assume is 10ms^-2), and h is the height of the object.

Substituting the values in:
GPE = 70*10*10 = 7000J