25
1) We must start by reminding the binomials. Since this question refers to a perfect square trinomial then we can write:

2) Now we can look at the expression. Let's start by dividing that 10 by 2, since we have to write that -10x as 2 times the product of x and the other term:

Note that the second term is 5²= 25
3) Hence, the answer is 25