Pythagorean Theorem says that a^2 + b^2 = c^2 in 2 dimensions. However, it is also true for 3 dimensions. L^2 + W^2 + H^2 = D^2 where length = L, width = W, height = H, and diagonal = D.
Solve for H.
H^2 = D^2 - L^2 - W^2
H^2 = 7^2 - 6^2- 2^2 = 9
H = sqrt(9) = 3
The height is 3 feet