I am working with derivatives and the problem is:

The flight of a spaceship is described by the curve y = x^3 - 8x, x> 0. if the engine is shut off at point P, the space ship will fly off along the tangent line at P. If the spaceship is moving left to right, at what point should the engine be shut down in or to reach the point (4,0)

I know dy/dx is 3x^2 - 8

I'm not getting the answer though. not sure what I am missing, I know I should know how to solve this but it is escaping me at the moment. help would be greatly appreciated.

thank you!