3) (15a²b³ - 15abc²d) - (6ab²cd + 6c³d²) = 15ab(ab² - c²d) - 6cd(ab² - c²d) =
= (15ab - 6cd)(ab² - c²d) = 3(5ab - 2cd)(ab² - c²d).
4) (x³ +ax²) + (abx + bx²) + (acx + cx²) + (abc + bcx) =
= x²(x+a) + bx(x+a) + cx(x+a) + bc(x+a) = (x+a)(x² + bx + cx + bc)=
= (a+x)·(x(x+b) + c(x+b)) = (a+x)(x+b)(x+c).