Powerful is a synonym of strong - can you think of any more synonyms of "powerful"?
How about: energetic, forceful, impressive, influential, mighty, robust or vigorous? Or what about: commanding, forcible, omnipotent, strapping or supreme? Do you know where to find synonyms? In a thesaurus! You probably have a thesaurus in your school library, but you can also look online whenever you are stuck for a word.
Remember to choose your synonym carefully, however, because they don't all share the exact meaning. "Strapping" refers to being physically strong and sturdy. This is not the same aspect of "powerful" conveyed by "omnipotent". "Omnipotent" literally means all-powerful, which is beyond any description of a human being and is in fact reserved as a description of divine power. "Influential", on the other hand, does not refer to physical power at all, instead being used to convey political, societal, or another persuasive power. Try to be aware of the different shades of meaning each synonym has.
This second English synonym quiz will help to build your vocabulary. Why settle for one word when there are many more just waiting to be used! Try to use synonyms in your speaking and writing.