a) 3x² + 5x - 3y² - 5y
= (3x² - 3y²) + (5x - 5y)
= 3(x² - y²) + 5(x + y)
= 3(x + y)(x - y) + 5(x + y)
= (x + y)[3(x - y) + 5]
= (x + y)(3x - 3y + 5)
b) 3x(3 - x) - 6(x - 3)
= 3x(3 - x) + 6(3 - x)
= (3 - x)(3x + 6)
= 3(3 - x)(x + 2)
c) x² + 4x - 21
= x² + 4x + 4 - 25
= (x² + 4x + 4) - 25
= (x + 2)² - 5²
= (x + 2 - 5)(x + 2 + 5)
= (x - 3)(x + 7)