To find the surface area of a pyramid the formula is:
SA - Surface Area
B - Base
n - number of sides of the pyramid not includung the base
b - length of one side of the base
l - slant height
SA = B + 1/2 nbl
Since we know everything but the "l" we can solve for "l". I will replace "l" with x to avoid confusion.
336 = 144 + (4 * 12 * x) / 2
336 - 144 = 144 - 144 + (4 * 12 * x) / 2
192 = (4 * 12 * x) / 2
192 = 48x / 2
192 = 24x
192 / 24 = 24x / 24
x = 8
Slant Height = 8 inches