A source of sound emitting a 1200 Hz note travels along a straight line

A source of sound emitting a $1200 \mathrm{~Hz}$ note travels along a straight line at a speed of $170 \mathrm{~m} \mathrm{~s}^{-1}$. A detector is placed at a distance $200 \mathrm{~m}$ from the line of motion of the source. (a) Find the frequency of sound receive by the detector at the instant when the source gets closest to it. (b) Find the distance between the source and the detector at the instant in detects frequency $1200 \mathrm{~Hz}$. Velocity of sound in air $=340 \mathrm{~m} \mathrm{~s}^{-1}$.