Answer:

3x² + 4xy + 3y³

Step-by-step explanation:

(3x² + 8xy + 3y² ) + ( -3y² - 4xy + 3y³)

Get rid of the unnecessary parentheses

3x² + 8xy + 3y² - 3y² - 4xy + 3y³

Organize the similar terms

3x² + (8 - 4) xy + (3 - 3) y² + 3y³

Arrange the constant term

3x² + 4xy + (3 - 3) y² + 3y³

Organize the mononomial expression

3x² + 4xy + 0 + 3y³

0 does not change when you add or subtract

3x² + 4xy + 3y³