A small plane is 34 miles east of an airport runway (at the origin) and starts to descend to the airport according to the formula h = -.00034x^3 +.0173 x^2 +.098x where h is the height of the plane and x is the distance of the plane from the runway. How far from the runway (to the nearest mile) would the plane be descending at the greatest rate?

I started by taking the second derivative, and am not even sure if that is right, any help is appreciated.