If f(x) = x³ − x − 24, for which value of x is f(x) = 0?

Correct answer: b 3

f(3) = 27 − 3 − 24 = 0.

Why the other options are wrong

• a 2: f(2) = 8 − 2 − 24 = −18, which is not zero, so 2 is not a root of f; you must try a bigger value.
• c 4: f(4) = 64 − 4 − 24 = 36, not zero; f is already zero at 3, so 4 goes past the root.
• d 6: f(6) = 216 − 6 − 24 = 186, not zero; f is already zero at 3, so 6 is far too large.