write (x-4y) ^2 as a polynomial
(x - 4y)²(x - 4y)(x - 4y)x(x - 4y) - 4y(x - 4y)x(x) - x(4y) - 4y(x) + 4y(4y)x² - 4xy - 4xy + 16y²x² - 8xy + 16y²