(y - 6)² - 9y² = (y - 6)² - (3y)² = (y - 6 - 3y)(y - 6 + 3y) = (- 2y - 6)(4y - 6) =
= - 2 * 2 * (y + 3)(2y - 3) = - 4(y + 3)(2y - 3)
c² - d² - c + d = (c² - d²) - (c - d) = (c - d)(c + d) - (c - d) = ( c -d)(c + d - 1)
x4=1/3*x3=1/3*1/3*x2=(1/3)^3*x1=1/27*(-9)=-1/3