What method did you use (sorry if you said this above already, I haven't followed this thread too closely)? If you did the parabola method I mentioned before, it's self-evident really. Plus the question's wording makes it sounds like they let you assume it (if I recall the question correctly).
Looking back at the Q. now, it's two marks. So at least you should have some working out, and the answer. It'd probably be best to justify when the maximum occurs (e.g. concave down parabola). If you use this parabola method, you don't need to test, just need to do something like mention it's a concave down parabola, so it's maxed at the vertex etc.