So some of the permutations would be ABC ACB BAC BCA CAB and CBA. Displaystyle text Cleft nrrightfrac n rleft n-rright Cn r. The possible row or column permutations form a group isomorphic to S3 S3 of order 3.
Its just a standard notation used for combinations but you might also see something like nCr used instead.
The rest of the formula is more straightforward. 20922789888000 6 6227020800. Combinations of abcdefg that have at least 2 of ab or c. The number of combinations is the number of ways to arrange the people on the chairs when the order does not matter.