We have
f(x) = x3 -6x2 +11x -6 and g(x) = x2 -3x+2
g(x) = x2 -3x+2 = (x-1) (x-2)
Clearly (x-1) and (x-2) are factors of g(x)
In order to find whether g(x) = (x-1) (x-2) is a factor of f(x) or not, it is sufficient to prove that (x-1) and (x-2) are factors of f(x) .
i.e., we should prove that f(1) = 0 and f(2) = 0