Answer
$7x(2x+5)^2$
Work Step by Step
For the polynomial
$$
28x^3 +140x^2 +175x=7x(4x^2+20x+25)
$$
one can see that the polynomial $(4x^2+20x+25)$ is the expansion of a perfect square. We can thus use the formula:
$(a+b)^2=a^2+2ab+b^2$
That is
$$
28x^3 +140x^2 +175x=7x(4x^2+20x+25)=7x(2x+5)^2
.$$
