Please help me

Mobistar is a mobile services company that sells 800 phones each week when it charges 80 per phone. It sells 40 more phones per week for each $2 decrease in price. The company's revenue is the product of the number of phones sold and the price of each phone. What price should the company charge to maximize its revenue? Let d represent the number of $2 decreases in price. Let r represent the company's revenue. Write a quadratic function that reflects the company's revenue.

Part B: Find the vertex of the quadratic function above. How will finding the vertex help you determine at what price the company should charge to maximize its revenue?

Part C: Graph this function and show in the graph what price should the company charge.