a)
\(\left(y^3+8\right)+\left(y^2-4\right)\)
\(=\left(y+2\right)\left(y^2-2y+4\right)+\left(y-2\right)\left(y+2\right)\)
\(=\left(y+2\right)\left(y^2-2y+4+y+2\right)\)
\(=\left(y+2\right)\left(y^2+y+6\right)\)
b)
\(x^6-1=\)
\(=\left(x^3+1\right)\left(x^3-1\right)\)
\(=\left(x+1\right)\left(x^2-x+1\right)\left(x-1\right)\left(x^2+x+1\right)\)