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! :)