= Solution
At a single interaction insertion, the <Dyson series> supplies $i\mathcal L_{\rm int}=-ig\varphi^\dagger\varphi\phi$. Amputating the three external propagators in <6/b> gives the momentum-space rule
$$
\boxed{\varphi^\dagger\!\text{--}\varphi\!\text{--}\phi\ \text{vertex}:\quad -ig.}
$$
If the momentum-conservation factor is included in the vertex convention, append $(2\pi)^4\delta^{(4)}(p_\phi+p_\varphi+p_{\varphi^\dagger})$, with all momenta incoming. There is one charged line entering and one leaving the vertex, and a neutral real-scalar line. The absence of a factorial in the interaction is appropriate because its three fields are distinct. The amputated decay amplitude has magnitude $|g|$ with the usual convention that a graph computes $i\mathcal A$.
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