Arock with a weight of 156.8 newtons falls 5 meters. what potential energy does it have just before the end of its fall.

The formula for GPE is gpe=mgh, where m is the mass in kilograms, g is the acceleration due to gravity, and h is height above the ground in meters. The reason you multiply the mass by the gravitational field strength, is to get the weight in newtons, which you have already been given, hence, you would times the weight of 156.8 by 5, which then gives you 784J.