What the statement means is that x^2 + kx + 2k = 0 when x = -3. That's because x + 3 will equal zero (-3)^2 + (-3k) + 2k = 0 9 - 3k + 2k =0 9 - k = 0 9 = k
Check ===== x^2 + 9x + 18 = 0 (x + 3)(x + 6) = 0 These are the two factors. As expected from the givens one of them is (x + 3)