Given that $\displaystyle y=x^2-x$, $\displaystyle {dx}/{dt}>0$ and that the particle is traveling at a constant $\displaystyle 2\sqrt{10}$, what is dy/dt?

So far I've used a formula for speed as $\displaystyle \sqrt{(dy/dt)^2+(dx/dt)^2}$ and set it equal to (40)^(1/2). Any help would be hot.

Thanks.