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1. when the rubber band is stretched we know that the rubber band would gain the energy called " potential energy ". we know that a object would gain potential energy because of its shape or position because there is a temporary change in its shape. 

2. when we release the rubber band back then the rubber band would gain the kinetic energy. we know that kinetic energy is gained by a object due to it's motion. 

3. this total stretching and the coming back to it's normal position the rubber would gain it's mechanical energy.
⇒ potential energy + kinetic energy = mechanical energy 
the point 1 would be your answer and the rest would be the next stages
hope so my answer is useful and mark as best if possible :D