Two submarines are approaching each other in a calm sea.

Two submarines are approaching each other in a calm sea. The first submarine travels at a speed of $36 \mathrm{~km} \mathrm{~h}^{-1}$ and the other at $54 \mathrm{~km} \mathrm{~h}^{-1}$ relative to the water. The first submarine sends a sound signal (sound waves in water are also called sonar) at a frequency of $2000 \mathrm{~Hz}$. (a) At what frequency is this signal received from the second submarine. At what frequency is this signal received by the first submarine. Take the speed of the sound wave in water to be $1500 \mathrm{~m} \mathrm{~s}^{-1}$.