Volume because of weight and surface
If this is an equilateral triangle, we need half of it to find the height which is a leg of a right triangle with a hypotenuse of 15. That means that the base of this half triangle is 7.5. Use Pythagorean's Theorem to fine the height.

. x = 12.99 or 13
It equals 8 1/8 because 5 + 3 = 8 plus the 1/8