I NEED HELP PLEASE, THANKS! :) Compare and contrast electric potential energy and electric potential difference?

Calculate the speed of a projectile to determine how to put an object in orbit. here is the information you require: the projectile ’ s mass is not needed. the projectile must be launched on the horizontal plane. the earth drops 8 inches for every mile traveled. the acceleration due to gravity is 9.81 meters per second squared. ignore air resistance. you will have to calculate how far an object drops every second due to gravity. can anyone show me how to get the calculations for this? i can do the rest of the assignment but i can't get these first set of numbers right and it's throwing everything off!