Could someone teach me how to solve this?A large pizza franchise delivers large pizzas for $16. The equation y=-3x^2+72x+480 describes the profit, y, based on the increase, x, in the price of their large pizzas. Determine the range of prices, to the nearest cent, for which the franchise could sell each pizza and still make a profit of at least $640.

PS: Knowing why your answer is correct will help me more than just knowing the answer, so please explain why your answer is correct.