The apparent frequency is maximum when relative velocity of approach of tuning fork with respect to observer is maximum.
\(O P=\sqrt{17^2-8^2}=15 \,m\)
$ {z_1} = A\sin (kx - \omega \,t) $ , $ {z_2} = A\sin (kx + \omega \,t) $ , $ {z_3} = A\sin (ky - \omega \,t) $ .