Answer:
As you said you already know, energy cannot be created or destroyed.
Step-by-step explanation:
You cannot gain energy or lose energy, it can only be converted. So if you start on a 3m high hill and go down it, your potential energy is equal to mgh, and if you get to the bottom of the hill, your KE would be equal to your PE at the top, and when you start going up another hill again, the maximum height you can reach is 3m, because energy cannot be created or destroyed, and your mass and gravitational acceleration are the same, so therefore you can only reach the same height you started from due to the conservation of energy.