Answer:
This is the sum ---> 4x^2y + 4xy^2 - 1
Explanation:
(3x^2y+4xy^2-2y^3+3) + (x^2y+2y^3-4)
(remove the parenthesis)
= 3x^2y + 4xy^2 - 2y^3 + 3 + x^2y + 2y^3 - 4
(now, group the like terms)
= -2y^3 + 2y^3 + 4xy^2 + 3x^2y + x^2y + 3 - 4
(add the elements you think are alike or similar)
= -2y^3 + 2y^3 + 4xy^2 + 4x^2y + 3 - 4
= 4xy^2 + 4x^2y + 3 - 4
= 4xy^2 + 4x^2y - 1