A + B = 5x² - 2xy + 6x² + 9xy - y² = ( 5x² + 6x² ) + ( 9xy - 2xy ) - y² = 11x² + 7xy - y²
A - B = 5x² - 2xy - ( 6x² + 9xy - y² ) = 5x² - 2xy - 6x² - 9xy + y² = ( 5x² - 6x² ) - ( 2xy + 9xy ) + y² = -x² - 11xy + y²
B - A = 6x² + 9xy - y² - ( 5x² - 2xy ) = 6x² + 9xy - y² - 5x² + 2xy = ( 6x² - 5x² ) + ( 9xy + 2xy ) - y² = x² + 11xy - y²