Can someone explain the law of conservation of energy?

The law of conservation of energy is a law that states that "energy cannot be created or destroyed, only changed" this means that you can't create it or destroyed as in, if you burn wood, you have released the energy. you haven't destroyed it, you have just released it

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