When we find the square root of a number we get 2 options, the positive and the negative one, for example:
[tex]\begin{gathered} \sqrt{16}=4 \\ or \\ \sqrt{16}=-4 \end{gathered}[/tex]Therefore, Ryan made an error in the last step because x is equal to 6 or x is equal to -6:
[tex]\begin{gathered} x^2=36 \\ x=\sqrt{36} \\ x=6 \\ or \\ x=-6 \end{gathered}[/tex]