f(x + 1) = x2 - 3x + 2 ---> f(x + 1) = (x - 2)(x - 1) ---> f(x) = (x - 3)(x - 2)
f(x) = 0 ---> (x - 3)(x - 2) = 0 ---> x = 2 or x = 3
Sum = 2 + 3 = 5
That is another really tricky little question.
thanks Geno :)