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 }