here's the problem;

"Suppose an observer is on top of a skyscraper that is 1000 ft. If a car is moving away from the base of the skyscraper at a constant speed of 40 ft/sec, how fast is the distance from the observer to the car increasing when the car is 500 ft from the skyscraper?"

so far, i have the equation $\displaystyle a^2 = 1000^2 + c^2$ which i was going to differentiate but i dont know if thats right. please someone help me to do this..