Achild on a sled starts from rest at the top of a 15â° slope. if the trip to the bottom takes 23.9 s how long is the slope? assume that frictional forces may be neglected.

The first thing you should do for this case is a free body diagram to find the force acting on the body in the direction of the slope.
 We have then:
 F = m * g * sin (15)
 Also, by definition:
 F = m * a
 Matching:
 a = g * sin (15) = 9.80 * sin (15) = 2.536 m / s ^ 2
 Then, the kinematic equation for the final position in direction x is:
 x = vx0 * t + 1/2 * a * t ^ 2
 vx0 = 0 (Initial rest)
 x = 1/2 * 2.536 * (23.9) ^ 2 = 724.3m
 Answer
 the slope is 724.3m long

I'm pretty sure the answer is c