Physics(Please respond)
posted by Hannah .
1) David has a mass of 86.4kg and is skating with inline skates. He sees his 24.8kg younger brother up ahead standing on the sidewalk, with his back turned. Coming up from behind, he grabs his brother and rolls off at a speed of 2.99 m/s. Ignoring friction, find david's speed just before he grabbed his brother.
I do not know how to start this.

The law of conservation of linear momentum
m1•v = (m1+m2) •u
v= (m1+m2) •u/m1 =(86.4+24.8) •2.99/86.4 =3.85 m/s