7. [12 Points] A charter flight charges a fare of $300 per person plus $20 per person for each unsold seat on the plane. The plane holds 100 passengers. Let x represent the number of unsold seats. (a) Find an expression for the total revenue received for the flight R(x). Plot the graph. You can use Desmos for your plot. (b) Find the number of unsold seats that will produce the maximum revenue. (c) Find the maximum revenue