Answer: 25
Step-by-step explanation: The number that is added to both sides is the number that is needed to create a perfect square trinomial.
The question is, what is that number?
Well, it comes from a formula. The number that is added to both sides always comes from half the coefficient of the middle term squared.
So half of -10 squared or -5 squared which is 25.
So 25 is added to both sides.
