I figured it out. Try using algebra and solve for L. Do this using this formula. 1/2p(L +a) = SA where p is perimeter, L is slant height, and a is apothem, and SA is your Surface Area. Then just fill in your numbers.
like this: p = 32 L=L and a = 4.8, SA= 288
fill it in: 1/2(32)(L +4.8) = 288
16L + 76.8=288
-76.8 of both sides and you get
16L = 211.2; then divide 16 from both sides to get.
L=13.2 then substitute it in your formula-1/2p(L+a)=288. Hope it works for you