Answer
A passenger in the oncoming boat hears a frequency of $346.5~Hz$
Work Step by Step
We can use the equation for the Doppler effect when both the observer and the source are moving toward each other:
$f_o = \left(\frac{v+v_o}{v-v_s}\right)~f_s$
$f_o = \left(\frac{343~m/s+15.6~m/s}{343~m/s-20.1~m/s}\right)~(312~Hz)$
$f_o = 346.5~Hz$
A passenger in the oncoming boat hears a frequency of $346.5~Hz$.
