Find a 95% confidence interval for the percentage of cars on a certain highway that have poorly adjusted brakes, using a random sample of 500 cars stopped at a roadblock on that highway, 87 of which had poorly adjusted brakes.

I think I need to find k, with

$\displaystyle k = \frac{cs}{\sqrt n}$

n = 500, $\displaystyle \mu = 87$.

Not sure how to find c or s.