When you make the product of a binomial of the kind x + a times other binomial that is of the kind x - a, you obtain another binomial (not a trinomial), so any example with that form will be a counterexample that disproves the conjecture:
(x + a) * (x - a) = x^2 - a^2
For example, (x +3) * (x - 3) = x^2 - 9. So, not always the product of two binomials is a trinomial.
