(x-y)² is the factor of perfect square trinomial.
To find its product (the perfect square trinomial):
(x-y)² = x² - 2xy + y² ⇒ ANSWER
Remember that the product of square of binomial is a perfect square trinomial. Hence, the patter:
(x-y)² = x² -2xy + y²
(x+y)² = x² + 2xy + y²