Find the surface area of a regular square pyramid with a base edge length of 9 feet and a slant height of 12 feet.

Perimeter = 9 . 4=36 ft

lateral area=(perimeter . slant height) / 2 = (36 . 12) /2 = 216 sq ft

area of the base=9*9=81 sq ft

Surface area = LA+BA = 216 + 81 = 297 sq ft

we have three similar triangles, because each has a right angle and shares an angle.   let's write the angles in order: opposite to short leg, long leg, hypotenuse.

cab similar to bad similar to cbd

or as ratios,

ca: ab: cb = ba: ad: bd = cb: bd: cd

we also know

ac = ad + cd

(ad+cd): ab: cb = ba: ad: bd = cb: bd: cd

(ad+cd)/cb=cb/cd

we have cb=6, ad=5 and seek x=cd.

we reject the negative root and conclude x=4

answer: 4