Which of the following is not a combination?

the number of ways a president and vice president can be chosen from a group of 10 people.
the number of 2 person subcommittees that can be chosen from a 10 person committee.
the number of 2 person teams that can be picked form 10 people.

The answer is the number of ways a President and Vice President can be chosen from a group of 10 people.

When you're picking people for specific roles, the order in which people are picked matters. That is a permutations situation, not combinations.

For the others, they're situations that can be modeled with combinations since order does not matter. All that matters when you make a subcommittee or a duo-team is who's in it.

First subtract 10 from both side
y^2+6y-16=0
(y+8)(y-2)=0
-8 and 2