Law of Conservation ?'s

This is a Law of Conservation of Energy problem.

It says, a rock is dropped from the top of a 100 ft. building. Find the velocity of the rock when it's 75 ft. off the ground.

This is all the info I'm given. I feel like I'm needing the mass of the rock to solve this but don't know how to find it. Or is the info I'm given enough? I'm lost on this one.

Law of Conservation ?'s

Just use energy conservation law

Energy at top = Kinetic energy at 75 feet + potential energy at 75 feet

mg*100 = mv^2 / 2 + mg*75

Just equate this and get value of velocity v