Which situation describes a system with decreasing gravitational potential energy?

An example of a system with decreasing gravitational potential energy is a ball in free fall. 

At its initial height h, the ball has a gravitational potential energy equal to

where m is the ball mass and g is the gravitational acceleration. As the ball falls toward the ground, its height h decreases, and so its gravitational potential energy decreases as well, according to the formula 

where h' is the new height of the ball.

Distance^2= yellow line ^2 (see picture) = 30^2 + 49^2so distance = squareroot of (300^2 +49^2)i dont know if this is how to solve it, but i cnt think of another way