Look at the picture.
It's the isosceles triangle.
Use the Pythagorean theorem:
[tex]x^2=a^2+a^2\\\\x^2=2a^2\\\\x=\sqrt{2a^2}\\\\x=a\sqrt2[/tex]
Therefore:
If you are knowing the length of side a, then [tex]x=a\sqrt2[/tex].
If you are knowing the length of side x, then [tex]a=\dfrac{x\sqrt2}{2}[/tex]
