A sound source, fixed at the origin,

A sound source, fixed at the origin, is continuously emitting sound at a frequency of $660 \mathrm{~Hz}$. The sound travels in air at a speed of $330 \mathrm{~m} \mathrm{~s}^{-1}$. A listener is moving along the lien $x=336 \mathrm{~m}$ at a constant speed of $26 \mathrm{~m} \mathrm{~s}^{-1}$. Find the frequency of the sound as observed by the listener when he is (a) at $y=-140 \mathrm{~m}$, (b) at $y=0$ and (c) at $y=140 \mathrm{~m}$.