one algebra question

a) A statement defining any unknown quantities in the problem in terms of variables.

b) An equation involving the variable(s) corresponding to the information given in the problem.

c) A complete solution to the equation, showing all work.

d) A complete answer to the question asked, including appropriate units.

7) The stopping distance, d, of a particular car after the brakes are applied varies directly as the square of the rate, r. If the car is travelling 40 mph, it can stop in 80 feet. How many feet will it take the same car to stop when it is travelling 80 mph?