Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$5x(25x^2+15x+3)$
Since the polynomial $(5x + 1)^3 - 1$ is a difference of two cubes, we have $$(5x + 1)^3 - 1=(5x+1 -1)((5x+1)^2+(5x+1)+1)\\ =(5x)(25x^2+10x+1+5x+1+1)\ =5x(25x^2+15x+3) .$$ where we used the formula $(x-y)^3=(x-y)(x^2+xy+y^2)$. The polynomial $25x^2+15x+3$ can not be factored further since it is prime.