✔ 最佳答案
See this :
9-9y²-4x²+12xy
= 3² - 9y² - 4x² + 12xy
= 3² - 9y² - ( +4x² ) - ( -12xy ) <--- 負負 = 正 , 負正 = 負
= 3² - ( 9y² + 4x² - 12xy )
= 3² - ( 3y + 2x ) ²
= [ 3 + ( 3y + 2x ) ] [ 3 - ( 3y + 2x ) ] <--- a² - b² = ( a + b ) ( a - b )
= ( 3 + 3y + 2x ) ( 3 - 3y - 2x )