I am way stumped. I know how to find combinations, but I have no clue how to find them with the combination given.

To furnish a room, a decorator is to select 2 chairs and 2 tables from a collection in a warehouse that are all different from each other. If there are 5 chairs in the warehouse, and 150 different combinations are possible. How many tables are in the warehouse?

The correct answer is 6, but I can't figure out how to get there. I can't even seem to plug in the correct answer to my calculations to reach the number 150.