Find the Value of X - what tool is best?

My wife likes to solve puzzles and found this one a few days ago.

Find X in this equation:

Sqrt(x+15)+Sqrt(x)=15

She worked a bit on the problem and failed. She brought it to me as I once taught math in high school.

The first problem is to find X, but the bigger question is what method did you use to solve it?

If you are up to the challenge, could you find X and then tell me how you solved it!!

I'll give you my method after I get a few replies.