Calculating both final velocities (conservation of momentum)

How do you calculate both final velocities in an elastic collision (assuming all momentum is conserved). For example, if a 1000kg car going at 20m/s rammed into a 800kg car going at 30m/s opposite direction, what would both their final velocities be?



In my year 11 physics course, we only covered questions that gave you 3 of the 4 velocities, so I am interested as to how you are meant to figure this type of question out.



Thanks for any help.

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