Let's denote this as $M_1$. We know that the mass of ${ }_{19}^{39} \mathrm{~K}$ is 38.963708 atomic mass units (amu) and the mass of a neutron is 1.008665 amu. So, we have:
\[M_1 = 38.963708 \, \text{amu} + 1.008665 \, \text{amu} = 39.972373 \, \text{amu}\]
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