You're trying to solve for x. So, to get x by itself, you need to divide by 9 on both sides and when you divide -108 by 9, you end up with x equaling -12.
A triangular pyramid has 4 triangular faces. If all 4 have the same area, then one of them has area 1/4 of the surface area, 1836. That gives 459 sq ft as the area of one of the triangular faces.
