(f g)(x) means f(x) * g(x), so plug the value of each function in:
(x+2)(x-4)
And solve by distributing using FOIL:
Firsts: x * x = x²
Outers: x * -4 = -4x
Inners: 2 * x = 2x
Lasts: 2 * -4 = -8
Combine them all together:
x² - 4x + 2x - 8
Combine like terms (the -4x and 2x):
x² - 2x - 8
So the second to last one is correct.