The Hoover Dam is 726 ft high. Find the potential energy of 1.00 million ft3 of water

Question

at the top of the dam.(1 ft3 of water weighs 62.4 lb.)

Answer 1

You are mixing weight with mass.
PE=weight*height=density*volume(height)
= 62.4 lbs/ft^3*1E6 ft^3*726ft= you do it (this is in ft-lbs of energy)

Answer 2

Did it like this
1.00E6*62.4*736=4.53ftlb