Думаю, что так:
b³-8c³= b³-2³c³= (b-2c) ( b²+2bc+4c²)
49x²y- y³ = y(7²x²-y²)=y (7x-y)(7x+y)
-7a²+14a-7 = -7 (a²-2a+1) = -7*(a-1)²
5ab-15b-5a+15=(5ab-15b) + (15-5a)=
=5b (a-3) +5 (3-a)= 5b(a-3)-5(a-3)=
=(a-3)(5b-5)= 5(b-1)(a-3)
a⁴-1 = (a²)²-(1²)²= (a²-1)(a²+1) = (a-1)(a+1)(a²+1)