a) \(x^3+6x+20\)
\(=x^3-2x^2+10x+2x^2-4x+20\)
\(=x\left(x^2-2x+10\right)+2\left(x^2-2x+10\right)\)
\(=\left(x+2\right)\left(x^2-2x+10\right)\)
b) \(x^3-7x-36\)
\(=x^3+4x^2+9x-4x^2-16x-36\)
\(=x\left(x^2+4x+9\right)-4\left(x^2+4x+9\right)\)
\(=\left(x-4\right)\left(x^2+4x+9\right)\)
c) \(3x^3-7x^2+17x-5\)
\(=3x^3-6x^2+15x-x^2+2x-5\)
\(=x\left(3x^2-6x+15\right)-\frac{1}{3}\left(3x^2-6x+15\right)\)
\(=\left(x-\frac{1}{3}\right)\left(3x^2-6x+15\right)\)