Answer
$f'=526$ Hz
Work Step by Step
To find the apparent frequency of the tone, use the Doppler effect formula of $$f'=f(\frac{v \pm v_o}{v \mp v_s})$$ To find the maximum velocity, use the formula relating velocity and radius, which is $v=r\omega$. Substituting in values of $\omega=15.0rad/s$ and $r=.600m$ yields $$v=(.600m)(15.0rad/s)=9.00m/s$$ Substituting this value while the whistle is moving away from the observer yields $$f'=(540Hz)(\frac{343m/s}{343m/s + 9.00m/s})=526Hz$$
