Please help! I cannot figure it out. I've been working for a while

A company is increasing production of math-brain protein bars at a rate of 200 cases per day. All cases produced can be sold. The daily demand function is given by p(x) = 90 - 200' where x is the number of cases produced and sold, and p is in dollars. Find the rate of change of the revenue with respect to the time in days when the daily production is 300 cases. 17,600 dollars per day 18,400 dollars per day 17,400 dollars per day 17,800 dollars per day 18,300 dollars per day