You and a friend each hold a lump of wet clay. Each lump has a mass of 20 grams. You each toss your lump of clay into the air, where the lumps collide and stick together. Just before the impact, the velocity of one lump was < 5, 4, -3 > m/s, and the velocity of the other lump was < -2, 0, -6 > m/s.

What is the momentum of the stuck-together lump just after the collision?

What I did first was take the total mass of the two lumps and multiplied it to the sum of their velocities.... Can someone please explain to me what I did wrong??