Doppler effect physics question?

Two submarines are underwater and approaching each other head-on. Sub A has a speed of 19 m/s and sub B has a speed of 13 m/s. Sub A sends out a 1870 Hz sonar wave that travels at a speed of 1522 m/s. (b) Part of the sonar wave is reflected from B and returns to A. What frequency does A detect for this reflected wave?