How to find whether a grammar is ambiguous?
Since you are trying to prove that the grammar is ambiguous, you must simply provide an example of a string where that grammar results in more than one parse tree or derivation. (Note that this is an entirely different ordeal than trying to prove that a grammar is unambiguous.) Since the string abbbb indeed has two distinct leftmost  derivations, you have shown that the grammar is ambiguous. This proves that the grammar is not unambiguous since it is not the case that each string has at most one parse tree or derivation.