Answer
$25$, $(x+5)^2$
Work Step by Step
To complete the square of $x^2 + 10x$, we add $(b/2)^2$. In this case, the "$b$" is the second coefficient, $10$. Thus, we add $(10/2)^2=25$, that is, we have $x^2 + 10x+25$. Then by factorization, we get
$$
x^2 + 10x+25=(x+5)^2
.$$
