Lời giải:
a)
$70a-84b-20ab-24b^2=2(35a-42b-10ab-12b^2)$
b)
$12y-9x^2+36-3x^2y$
$=(12y-3x^2y)+(36-9x^2)$
$=3y(4-x^2)+9(4-x^2)$
$=(3y+9)(4-x^2)=3(y+3)(2-x)(2+x)$
c)
$21bc^2-6c-3c^2+42b=3(7bc^2-2c-c^2+14b)$
d)
$30a^3-18a^2b-72b+120a$
$=(30a^3+120a)-(18a^2b+72b)$
$=30a(a^2+4)-18b(a^2+4)$
$=(a^2+4)(30a-18b)=6(a^2+4)(5a-3b)$
P/s: Cảm giác câu a,c bạn viết sai đề.