Image attached. Pl help. Question is MCQ and related to motion. Correction 12 m/s

Answer is 15m/s. Here centre of mass of all three parts must be at rest.so the third part must move in a direction 135degree to the other part in opposite direction. The speed must resultant of other two parts =root of (12^2+9^2)=15m/s. Thank☺☺☺☺☺☺☺