A frictionless track is to be built as shown, with L= 4.7 m and H= 4.6 m. In order to get the cart to slide from the top to the end of the track in the minimum time, how long should the distance D be? Assume that the speed of the cart on the horizontal surface is the same as at the bottom of the ramp.

http://loncapa.gwu.edu/res/msu/physicslib/msuphysicslib/11_Force_Motion_Adv/graphics/prob05_ramp2.gif

The image doesn't work. I seems that one needs to sign in to be able to see the image...

And I have no idea how you can change D without changing L and H, but then, I'm certainly not understanding how the ramp is.