x² = 6400 

\(\Rightarrow x=\pm\sqrt{6400}\)

\(\Rightarrow x=\pm80\)

* x = 80 => x isn't a square number

* x = -80 < 0 isn't  square number

So................

We have : x² = 6400

=> x² = \(\pm80^2\)

To try on:

- x = 80 => x it isn't a square number

- x = -80 < 0 it isn't square number

=> x it isn't square number