Hi! :)
In this case, we have to square both sides of the equation. When we square the left side, we will get x^2 + 81, because square root squared denies one another, and for the right side, we have to square the binomial, like this:
(x+10)^2 = x^2 + 20x + 100
So the correct answer is the last one : x^2 + 81 = x^2 + 20x + 100.
Hope this helps!
