JAMB 1993 · UME · Q12

Which of the following is a factor of $15 + 7x - 2x^2$ ?

Worked solution (try it first)

Take out $-1$ so the $x^2$ term is positive: $15 + 7x - 2x^2 = -(2x^2 - 7x - 15)$ .
Factorise: $2x^2 - 7x - 15 = (2x + 3)(x - 5)$ .
So $x - 5$ is a factor, option C.
Check: $x = 5$ gives $15 + 35 - 50 = 0$ .