Explanation

To make x² - xy + y² a perfect square, we need to add or subtract a term that will complete the square.

Recognize that:

x² - 2xy + y² = (x - y)²

However, the given expression is:

x² - xy + y²

To match the perfect square form, we need to subtract:

xy

So, subtracting xy will make it:

x² - 2xy + y² = (x - y)²