Roller coaster velocity at the bottom of an incline

Homework Statement

A roller coaster starts its descent with an initial speed of 4 m/s. it moves through a distance of 135 ft (41.1 m) along an incline that makes an angle of 40 degrees with the horizontal. Neglect friction and find its speed at the bottom of the incline.

See below. . .

The Attempt at a Solution

I have the answer, and a process:

http://img707.imageshack.us/img707/9573/picture2la.png [Broken]

However, I don't understand how Fx can possibly equal mg sin40. I don't understand how to set up my coordinate axis so this is possible. I don't even know what Fx is supposed to represent any more. .

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