(70а-84b)+(20ab-24b^2)=
14(5a-6b)+4b(5a-6b)=(5a-6b)(14+4b)=
2(5a-6b)(7+2b).
2) Тот же способ.
(21bc^2+42b)-(3c^3+6c)=
21b(c^2+2)-3c(c^2+2)=
(c^2+2)(21b-3c)=3(c^2+2)(7b-c).
3) (12y-3x^2y)+(36-9x^2)=
3y(4-x^2)+9(4-x^2)=(4-x^2)(3y+9)=
3(2-x)(2+x)(y+3). (4-x^2)