Consider a sheaf of abelian groupS $\mathscr F$ over a topological space $X$. If $\mathscr F'$ is a subsheaf of $\mathscr F$ (over $X$), then we can construct the quotient presheaf $\mathscr F/\mathscr F'$ in the following way:
$$(\mathscr F/\mathscr F')(U):=\mathscr F(U)/\mathscr F'(U)$$
Now I don't understand why it is true that $(\mathscr F/\mathscr F')_x=\mathscr F_x/\mathscr F'_x$ for every $x\in X$.
Notation: $\mathscr F$ is a presheaf (of abelian groups) over $X$ and $x\in X$, with the notation $\mathscr F_x$ I mean the stalk at $x$.