elementary set theory - $A$ is equipotent with $B$, $ain A, bin B$, then $A-{ a}$ is equipotent with $B-{ b}$

I am trying to prove the following equipotence
If $A$ is equipotent with $B$, $a\in A, b\in B$, then $A-\{ a\}$ is equipotent with $B-\{ b\}$

I know the easiest way would be to show that the function going from
$A-\{ a\}$ to $B-\{ b\}$ is bijective. But I have no idea on how to start. Any help wod be appreciated.