That's not correct. In general, you would have
[tex]x^2=\dfrac5{a^2}\implies\sqrt{x^2}=\sqrt{\dfrac5{a^2}}\implies |x|=\dfrac{\sqrt5}{\sqrt{a^2}}\implies |x|=\dfrac{\sqrt5}{|a|}[/tex]
Assuming [tex]a>0[/tex], you have [tex]|a|=a[/tex], so this turns into
[tex]|x|=\dfrac{\sqrt5}a\implies x=\pm\dfrac{\sqrt5}a[/tex]
