A book that weighs 5 newtons is sitting on a table 1.5 meters above the floor. How much gravitational potential energy does the book have? 6.5 joules
4.5 joules
7.5 joules
3.33 joules

Part (a): h=vit+1/2gt^2  this equation will solve the first part. you know initial vertical displacement h, initial vertical velocity vi, and acceleration due to gravity g. so you would simply time t for each ball. remember to change signs for directions! subtract one from the other for the difference.
part (b): vf^2=vi^2+2gh  this equation will solve for the second part. you know initial vertical displacement h, initial vertical velocity vi, and acceleration due to gravity g. simply solve for vf.
part (c): i won't even dive into this part. if you solved for the time differential, then this should follow from that!