a)
\(x^2-8x-2x+16=x\left(x-8\right)-2\left(x-8\right)=\left(x-8\right)\left(x-2\right)\)
b)
\(6x^2-15x+8x-20=3x\left(2x-5\right)+4\left(2x-5\right)=\left(2x-5\right)\left(3x+4\right)\)
c)
\(x^3+3x^2+3x+3x+1+3=\left(x+1\right)^3+3\left(x+1\right)=\left(x+1\right)\left(x^2+2x+4\right)\)