Answer:
(x -8y)²
Step-by-step explanation:
The terms with squares are perfect squares, and the "xy" term is double the product of the roots of those, so you recognize this as a "perfect square trinomial." Its two factors are equal.
x² -16xy +64y² = (x -8y)²
_____
It matches the pattern ...
(a +b)² = a² +2ab +b²
with a=x, b=-8y.
