I'm stuck on this math problem:

"The stopping distance d of an automobile is directly proportional to the square of its speed v. A car required 75 feet to stop when its speed was 60 miles per hour. Find a mathematical model that gives the stopping distance d in terms of its speed v"

I'm not sure how to approach this problem. I tried plugging converting the 60 mph to ft (318,800ft) and then plugging it into $\displaystyle D=KV^2$, and then solving for K to get the constant term needed to write the equation.

Whenever I solve K, I get "7.37946E-10", and plug it into $\displaystyle D=(7.37946E-10)(V^2)$, which is apparently wrong according to the program I am using. Any help?