Finding velocity and direction of unmeasured ball in elastic collision

[thereddevils]

Given two balls of the same mass of known velocities and known direction (before and after collision) , only one of the ball is not given its velocity and direction before collision and i am supposed to find them .

As for the velocity , just take the sum be4 and after collision and evaluate the missing one .

How about the direction ? Do i draw a vector diagram ?

 

[SaintsTheMeta]

To find the velocity remember Momentum and Energy are conserved.

Hard to understand from your question, but it sounds like they are both traveling different 2D or 3D directions before collision, thus when they collide you can remember that Momentum is conserved in the x, y, and z direction. so:
$$\Sigma$$p_x=$$\Sigma$$p'_x
$$\Sigma$$p_y=$$\Sigma$$p'_y
$$\Sigma$$p_z=$$\Sigma$$p'_z