What is the coefficient of x in the expansion of (3 + x)(1 − x)⁸?

Correct answer: c − 23

(3)(coefficient of x, −8) + (x)(1) = −24 + 1 = −23, so this is the value.

Why the other options are wrong

• a − 2: −2 does not come from a correct working. The right answer is − 23.
• b − 7: −7 comes from −8 + 1 without multiplying −8 by 3. The right answer is − 23.
• d − 25: −25 comes from −24 − 1; the x-term of (3 + x) is +1. The right answer is − 23.