Algebra

This isn't hard to do, we just have to set up an equation. 

First, let s be the sidenlength of a equation. 

$s^2 = 10s + 38 \\ s^2 - 10s = 38 \\ s^2 -10s + 25 = 38 + 25$

$(s - 5)^2 = 63$

$s - 5 = \sqrt{63}$

$s = \sqrt{63} + 5$

The other possible equation would get a negative value, but a sidlength can't be negative, so we reject that possibility. 

So we have $s = \sqrt{63} + 5$

Thanks! :)