whats square root of minus 2

There is no real answer because a positive number times a positive number must be positive and a negativ number times a negative number is also positive, so, if there is a number, it can't be either positive or negative.

However there are "imaginary" numbers that work (they are different; they aren't either positive nor negative):

√(-2)  =  √(-1)·√2  =  i√2                              The letter "i" is used to replace √(-1).