7x-7y+ x² -2xy + y² = 7( x-y) + (x-y)²
= (7 + x-y)( x-y)
3x² + 6xy + 3y² - 3z²= 3( x² + 2xy + y²- z²)
= 3[( x+y)² - z²]
= 3( x+y-z)(x+y+z)
(x-y)³ + (y-z)³+ (z-x)³= ( x-y + y-z)[ ( x-y)² - (x-y)(y-z) + ( y-z)²)] + (z-x)³
= (x-z)( x-y)² - (x-y)(y-z) + ( y-z)²) + (z-x)³
= (x-z)[( x-y)² - (x-y)(y-z) + ( y-z)² + (z-x)²]