Using the kinematic equation:
(final speed)^2 = (initial speed)^2 + 2 * acceleration * distance
(12)^2 = (0)^2 + 2 * acceleration * distance
The child and sled are accelerating under gravity which exerts a force down the hill, so acceleration = g = 9.8 m/s^2 in the "down" direction.
144 = 2 * 9.8 * "down" distance (height of the hill)
height of the hill = 7.35m