Algebra

Let's note something for this problem first. 

We are basically taking a look at the squares of numbers, and eeing the largest 3 digit number. 

Thus, let's test some integers. We find that

$31^2 = 961 \\ 32^2 = 1024$

This shows that 31 is the largest value squared. 

Thus, 961 is our answer. 

So we have $x=961$

Thanks! :)