Answer
This is because the product is: $$ a ^2 - b^2$$
Work Step by Step
We know that the multiplication of the sum and difference of two binomials gives:
$$ a ^2 - b^2$$
This is by definition a binomial since it has two terms. Thus, the product of the sum and difference of two binomials, given by $(a-b)(a+b)$, will always result in a binomial answer.
