L^2 = 63 + (L-3)^2 = 63 + L^2 -6L + 9
use the Pythagorean Theorem, hypotenuse (ladder length) squared = sum of squares of the other two sides (height and distance from bottom of ladder to the building)
L^2 terms cancel laving
0 = 72 -6L
L = 72/6 = 12 feet long = length of the ladder
12^2 = 63 + 9^2 = 63+ 81 = 144