If $\lvert\ker(\varphi)\rvert=p$ and $p$ is prime, then $G=\ker(\varphi)$

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I have a group $G$ and a random subgroup $H\leq G$. Also I have a homomorphism $\varphi\colon G\rightarrow H$. If $\lvert\ker(\varphi)\rvert=p$ and $p$ is prime can you help me how I can prove $G=\ker{(\varphi)}$? I try with the First Group Isomorphism Theorem but I could not do anything.

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This is not true, take $p:G\times G\rightarrow G$ the projection where the cardinal of $G$ is prime.