Let b and h represent the length of the side of the base and the slant height, respectively. Then the total surface area is
A = b² + 4*(1/2)bh
A = b² +2bh
Substituting the given numbers, you have
336 = 12² +2·12·h
192 = 24h . . . . . . . . . subtract 12² = 144
8 = h . . . . . . . . . . . . . divide by 24
The slant height is 8 inches.
