SOLUTION
[tex]Let\text{ the third side \lparen hypotenuse\rparen be x;}[/tex]
Applying Pythagoras theorem;
[tex]\begin{gathered} x^2=9^2+9^2 \\ x^2=81+81 \\ x^2=162 \\ x=\sqrt{162}=\sqrt{81\times2}=9\sqrt{2} \\ x=9\sqrt{2} \end{gathered}[/tex]
The length of the third side is;
[tex]9\sqrt{2}[/tex]
