The question really does just want you to take the whole expression for x given by the quadratic formula and substitute it into ax² + bx + c, and verify that it all cancels down to 0. It might seem like a nightmare at first, but just be careful with your algebra and you should be fine. You also technically have to do it twice - once for each if the +- possibilities - but the work is 95% the same either way, so it shouldn't cause too much extra grief.