Which equation represents the pythagorean theorem, which can be used to find the magnitude of resultant vectors?

r = a + b

r = a x b

r2 = a2 x b2

r2 = a2 + b2

A14 kg rock starting from rest free falls through a distance of 5.0 m with no air resistance. find the momentum change of the rock caused by its fall and the resulting change in the magnitude of earths velocity. earth mass is 6.0 * 10^24 kg. show your work assuming the rock earth system is closed.