Normal Subgroups and Quotient Groups · Easy problem

Let $\varphi: \mathbb Z_{15} \to \mathbb Z_9$ send $k \mapsto 3k \bmod 9$. By the First Isomorphism Theorem, $|\mathbb Z_{15}/\ker\varphi| = |\operatorname{im}\varphi|$. What is this common value?