To factorise you need to first list the factors of 24 (1, 2, 3, 4, 6, 8, 12, 24)
Pair these up in the following way:
1, 24 (1 x 24 = 24)
2, 12 (2 x 12 = 24)
3, 8 (3 x 8 =24)
4, 6 (4 x 6 =24)
Of these 4 pairs, only 3 + 8 = 11
Hence the answer is
(x + 3)(x + 8) = x^2 + 3x +8x +24 = x^2 + 11x +24
(If you so choose you can also use the quadratic formula, or several other methods)
