Prove that for an associative algebraic structure, the inverse of every invertible element is unique.

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I don't know how to start this prove. For a group, it's easy to prove. Like we can start with Let e be an identity element. Suppose a belongs to G(where G is a group) and b,b' belongs to G are the inverse of the element a. now ae=ea=b and ae=ea=b' But I don't know how to start for semigroup.

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The fact that $b$ is the inverse of $a$ means $$ ab=ba=e $$ and not that $ae=ea=b$.

If $b'$ is another inverse of $a$, then $$ b'=eb'=(ba)b' $$ Now continue using associativity.