Find Velocity After Glancing Collision: Antother Collision

[suspenc3]

Antother Collision!

A billiard ball moving with a speed of $$v = 2.2m/s$$ strikes an identical stationary ball a glancing blow. After the collision, one ball is found to be moving at a speed of $$v_2 = 1.1m/s$$ in a direction making a $$\theta = 60^o$$ angle with the original line of motion. Find the velocity of the other ball?

I don't really know where to start...Maybe conserving momentum?

 

[suspenc3]

Would I just do this:

$$1/2m_1v_1_i^2 +1/2m_2v_2_i^2 = 2(1/2m_1v_1_f^2) + 1/2m_2v_2_f^2$$

since all of the masses are the same they cancel out and since the second ball is stationary..it is 0

$$v_1_i^2 = 2v_1_f^2 + 2v_2_f^2$$
$$1/2(2.2)^2 = 2sin \theta(1.1)^2 + 2v_2_f^2$$
$$v_2_f = 1.53m/s$$

 

[Astronuc]

Would I just do this:
$$1/2m_1v_1_i^2 +1/2m_2v_2_i^2 = 2(1/2m_1v_1_f^2) + 1/2m_2v_2_f^2$$

Why is the kinetic energy term of m₁ doubled on the right hand side?

In an elastic collision, kinetic energy and momentum are conserved. Momentum is conserved in the x and y directions.