Answer: [D] -16x^4y + 4xy^4 + 16xy
Remove parentheses: (-a) = -a
-3xy^4 - 7x^4y + 7xy - 9x^4y + 9xy + 7xy^4
Group like terms
-7x^4y - 9x^4y - 3xy^4 + 7xy^4 + 7xy + 9xy
Add similar elements: -7x^4y - 9x^4y = -16x^4y
= -16x^4y - 3xy^4 - 7xy^4 - 16xy
Add similar elements: -3xy^4 + 7xy^4 = 4xy^4
= -16x^4y + 4xy^4 + 16xy