3: \(15x^3+29x^2-8x-12\)
\(=15x^3+30x^2-x^2-2x-6x-12\)
\(=\left(x+2\right)\left(15x^2-x-6\right)\)
\(=\left(x+2\right)\left(15x^2-10x+9x-6\right)\)
\(=\left(x+2\right)\left(3x-2\right)\left(3x+5\right)\)
5: \(x^3+9x^2+26x+24\)
\(=x^3+4x^2+5x^2+20x+6x+24\)
\(=\left(x+4\right)\left(x^2+5x+6\right)\)
\(=\left(x+4\right)\left(x+2\right)\left(x+3\right)\)