Use the Pythagorean theorem:
[tex](\hbox{one leg})^2+(\hbox{the other leg})^2=(\hbox{hypotenuse})^2 \\
6^2+6^2=x^2 \\
6^2 \times 2=x^2 \\
\sqrt{6^2 \times 2}=x \\ \sqrt{6^2} \times \sqrt{2}=x \\ 6\sqrt{2}=x
[/tex]
The length of the hypotenuse is 6√2 units.
