**Problem: **
Two cars start moving from the same point. One travels north at 60 mph and the other travels west at 25 mph. At what rate is the distance between the cars increasing two hours later?

**What I've Done:**
So I drew a right triangle like this:

http://i193.photobucket.com/albums/z.../diagram-2.png
I know I want to find $\displaystyle \frac{dx}{dt}$ at t = 2, and I know I have $\displaystyle \frac{dN}{dt}$ which is 60mph and $\displaystyle \frac{dW}{dt}$ which is 25mph.

I wrote the equation $\displaystyle N^2 + W^2 = x^2$. Is this correct? Am I doing this right?

Do I just differentiate this equation with respect to x, and then plug in the values for $\displaystyle \frac{dN}{dt}$ and $\displaystyle \frac{dW}{dt}$ ?

I really have no idea what I'm doing. I think I need t in my equation somewhere...

Help?