How do you work this out ?

Q2. For a large shopping center sales increase with the number of customers. The relationship is given by S = 1/%C2 + 17, where S is the value of sales in millions and C is the number of cus- tomers in thousands. A new marketing campaign guarantees that customers will increase with time following a function 0 = 3.1 + 0.11:2, where t is measured in months. Find the rate of increase in sales after 3 months. Round up the answer to two decimals. [12 marks]