Prove that a function $f$ from a set $A$ to a set $B$ is onto if and only if it is right cancellable

Prove that a function $f$ from a set $A$ to a set $B$ is onto if and only
if it is right cancellable

Prove that a function $f$ from a set $A$ to a set $B$ is onto if and only
if it is right cancellable.
I know that a function $f$ from a set $A$ to a set $B$ is injective if and
only if it is left cancellable.