Finding zeros (math homework)

[VEN]

here's my last question, I hope:

A pizza company's research shows that a 25 cent increase in the price of a pizza results in 50 fewer pizzas being sold. The usual price of $15 for a pizza results in sales of 1000 pizzas. The questions are a) Write the algerbraic expression that models the revenue for this situation. and b) what is the optimum Max value?

so far I wrote down on my sheet y=(15+.25x)(1000-50x) This models the revenue so I guess I answered the first question

I know I can solve the second question by using a table and writing down the points of the parabola until I reach the max value, but how do I do it using the algebraic method?