Angular Velocity of weighted plates

[randomss444]

Homework Statement

A 7kg plate with a radius of 1.4m is rotating at 63 rad/s around an axis perpendicular to its center. Another plate that is not rotating is dropped onto the first plate and their centers align. The second plate is 5kg, with a radius of .8m. What is the angular velocity of the two disks combined?

Homework Equations

The Attempt at a Solution

I am really confused with this problem.
What is my first step?

Thanks

 

[pgardn]

Homework Statement

A 7kg plate with a radius of 1.4m is rotating at 63 rad/s around an axis perpendicular to its center. Another plate that is not rotating is dropped onto the first plate and their centers align. The second plate is 5kg, with a radius of .8m. What is the angular velocity of the two disks combined?

Homework Equations

The Attempt at a Solution

I am really confused with this problem.
What is my first step?

Thanks

Has your class been discussing the conservation of angular momentum?