Answer
$x=\dfrac{5\sqrt{2}}{2}$
Work Step by Step
Using $a^2+b^2=c^2$ (or the Pythagorean Theorem), then
\begin{array}{l}\require{cancel}
x^2+x^2=5^2
\\\\
2x^2=25
\\\\
x^2=\dfrac{25}{2}
\\\\
x=\sqrt{\dfrac{25}{2}}
\\\\
x=\sqrt{\dfrac{25}{2}\cdot\dfrac{2}{2}}
\\\\
x=\sqrt{\dfrac{25}{4}\cdot2}
\\\\
x=\sqrt{\left( \dfrac{5}{2} \right)^2\cdot2}
\\\\
x=\dfrac{5}{2}\sqrt{2}
\\\\
x=\dfrac{5\sqrt{2}}{2}
.\end{array}
