The upper limit for the function f (x) = x3 - 3x 2 - 2x + 15 is

By the zero product property, either (x^2 - 4x + 4 = 0

which easily factors into (x-2) (x-2) and thus gives two roots of 2 and 2

or

(x^2 + 3x - 10) = 0 which also factors to give 2 and - 5

so all 4 roots : 2,2,2, - 5

and for your answer : { 2 and - 5 }