Now we need to find the height of the pyramide. Let's notice that we have a right triangle with a hypotenuse of 9 ft and its base would be the half of the length of the square (15ft/2 = 7.5 ft) and its height would be the height of the pyramide. Then we can use Pythagoream theorem:
c^2 = a^2 + b^2 ; c is the hypotenuse, a and b are the other sides of the right triangle (its height and base).
So c = 9 and b = 7.5. Then we solve for a like this:
9^2 = a^2 + 7.5^2 -----------> a = sqrt(81 - 56.25) = 4.97 ft