Inelastic collisions

Mass 1 is moving with an initial speed of v_1, which undergoes a completely inelastic collision with a stationary block mass 2. The two blocks then stick together and move at a speed v_2. The two blocks then collide inelastically with a third block mass 3. Assuming that the blocks slide without friction what is v_2/v_1: the ratio of the velocity v_2 of the two block system after the first collision, to the velocity v_1 of the mass 1 before the collision?

I started with 1/2(m_1)(v_1)^2=1/2*(m_1+m_2)(v_2)^2 and then solved for v_2/v_1 but I'm not sure that is the right equation. Any help would be great.
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