Chapter 9 - Solving Quadratic Equations - 9.4 - Solving Quadratic Equations by Completing the Square - Exercises - Page 512: 35

$b=\pm 10$

$25$ is a square of $5$. So $b$ should consist two times $5$ that is $10$. $x^2+10x+25$ is a perfect square trinomial $(x+5)^2$. Also here might be $(x-5)^2$ and in this case value of $b$ must be $-10$. So two possible values are $10$ and $-10$.