\left(16x+8x^{2}y-7xy^{2}+9y\right)+\left(5xy^{2}+10x^{2}y+x-7y\right)

= 8x^{2}y + 10x^{2}y - 7xy^{2} + 5xy^{2} + x + 16x - 7y + 9y - 7y

= 18x^{2}y - 2xy^{2} + x + 9y