Intermediate Algebra (6th Edition)

4$x^{3}$ + 4$x^{2}$ + 16xy - 9x + 18
(9xy + 4$x^{2}$ + 18) + (7xy - 4$x^{3}$ - 9x) = -4$x^{3}$ + 4$x^{2}$ + (9xy + 7xy) - 9x + 18 = 4$x^{3}$ + 4$x^{2}$ + 16xy - 9x + 18