I got number 6 incorrect, could you explain and work out the correct answer please?

\f6. A 3.0-kilogram steel ball whose initial velocity is 10/205 meters 10 013 per second and a 5.0-kilogram block of putty whose initial velocity is 30/60 meters per second slide on a frictionless - 1200 surface. as shown. The objects collide and the ball imbeds itself in 30 the putty. Find the velocity of the combined mass immediately 60 after impact. 21.7 m/s