How to prove a quadrilateral is a parallelogram with slope and distance?

If it is on a coordinate plane:
1. Mark opposite lines and find their slopes.
2. If you see that they are the same, that means that they are parallel. 
3. Now, you have to check if the opposite lines measure the same distance. If the distance is already given, then make sure it is the same. If it isn't, you can use the distance formula and verify if the opposite lengths are the same.

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