Solution
= Solution
Between launch and interception, $\Delta x=vt_{\rm out}$. The time-coordinate <Lorentz transformation> therefore gives
$$
\Delta t_B=\gamma_v\left(t_{\rm out}-\frac{v^2t_{\rm out}}{c^2}\right)
=\frac{t_{\rm out}}{\gamma_v},\qquad
\boxed{\Delta t_B=\frac{4a}{\sqrt7c}=1.512\,\mathrm{days}.}
$$
This is an elapsed coordinate time in B's <inertial frame>, not the travel time on A's clock. The latter would be $t_{\rm out}/\gamma_u=3a/(\sqrt7c)$.