 Two point charges of $30 \mathrm{nC}$ and $-40 \mathrm{nC}$ are held fixed on an $x$ axis, at the origin and at $x=72 \mathrm{~cm}$, respectively. A particle with a charge of $42 \mu \mathrm{C}$ is released from rest at $x=28 \mathrm{~cm}$. If the initial acceleration of the particle has a magnitude of $100 \mathrm{~km} / \mathrm{s}^2$, what is the particle's mass?
$2.2 \times 10^{-6} \mathrm{~kg}$