3x^3+7x^2-4=(3x^3+3x^2)+(4x^2-4)
=3x^2(x+1)+4(x^2-1)
=3x^2(x+1)+4(x+1)(x-1)
=(x+1)(3x^2+4x-4)
=(x+1)(x+2)(3x-2)