Answer
$(x-5)(x+2)(x-2)$
Work Step by Step
The product of a binomial sum and a binomial difference is: $(A+B)(A-B)=A^2-B^2$.
The square of a binomial sum is: $(A+B)^2=A^2+2AB+B^2$.
The square of a binomial difference is: $(A-B)^2=A^2-2AB+B^2$.
Hence here: $x^3-5x^2-4x+20=\\=x^2(x-5)-4(x-5)\\=(x-5)(x^2-4)\\=(x-5)(x^2-2^2)\\=(x-5)(x+2)(x-2)$