You know the numbers listed above are not factors of 12 because they cannot be multiplied by anything to equal twelve (ex. 5 cant be multiplied by anything to equal 12, the closest would be 5 x 3 which is 15, not twelve) so, 5, 7, 8, 9, 10, and 11 are not factors of 12 because they cannot be multiplied by another number to result in 12.