Mapping/Composition/Right surjective/Exercise

Let $L,M,N$ be sets, and let

$f:L⟶M\text{ and }g:M⟶N$

be mappings with their composite mapping

$g\circ f:L⟶N,x⟼g(f(x)).$

Show that if $g\circ f$ is surjective, then also $g$ is surjective.

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