y² - 9y + 20 = (y - 5)(y - 4)
p³ - 7p² - 21p + 27 = (p³ + 27) - (7p² + 21p) = (p + 3)(p² - 3p + 9) - 7p(p + 3) =
= (p + 3)(p² - 3p + 9 - 7p) = (p + 3)(p² - 10p + 9) = (p + 3)(p - 1)(p - 9)
9x² + 9ax² - y² + ay² + 6axy = (9x² - y²) + (9ax² + ay² + 6axy) =
= (3x + y)(3x - y) + a(9x² + 6xy + y²) = (3x + y)(3x - y) + a(3x + y)² =
= (3x + y)(3x - y + 3ax + ay)
<span> t^3+ 4 + t^2 +4t = 0.</span>