Calculate how much time the stone take to achieve the highest point of its trajectory:
0 - 10 = -10 • t
-10 = -10t
t = -10/-10
t = 1
So, it took 1 second to achieve the highest point of trajectory. When the stone achieves it, its have velocity 0 and begin to fall, and it takes also 1 second to return to the top of the platform, so it means that during 2 seconds of the motion. The stone achieves the same high as the platform with the velocity of 10 m/s
Now let's calculate the distance its traveled to achieve the ground:
