A trinomial is a polynomial with three terms. It can be determined if it is a difference of two squares when you factor it.
The resulting factor of a trinomial that is a difference of two squares is: a² – b² = (a + b)(a – b) or (a – b)(a + b)
You will notice that the middle term is missing. This means that the middle term zeroes out as a result of having the same number of different signs (+ and -)