2n+5...a couple is two people so you need to multiply the number of couples, n, by 2 to get the total number of dancers. Then add the total number of organizers.
The expression 12xyz - 45 is a binomial because there are two terms that is being separated by the operation '-'. 12xyz is one term; 45 is another term. That makes two terms. Binomial is a polynomial with 2 terms.
