If both of these are factors of something, that mean that that can divide both of these with out a remainder. Though there are infinite amounts of these, this kind of question is usually asking for the smallest one. One way to find this is to multiply the factors. Using F.O.I.L, we get a^4 + 4a^2 - 6a^2 - 24. This simplifies to a^4 - 2a^2 - 24. Since we cannot simplify any more this is the answer.
