Normally you would say what number when multiplied by itself equals 9
Well 3 x 3 = 9 and (-3) x (-3) = 9 so why is it not
The reason is the √ operator or instruction means only the positive one so the first one is correct. There is more but it is jargon that you do not need right now.
That is the same thing but it is bit less clear. Although you understand it remember that math is language. Soon your mind will begin to utilize the rules of this language and you will speak it better and better. But because it is a language you must speak it to other people. Not only must you understand it they must understand it when you speak it to them.