To find the missing leg, we use Pythagorean's Theorem
[tex]c^2=a^2+b^2[/tex]Where c = 6 and a = 3. Let's use these values to find b-
[tex]\begin{gathered} 6^2=3^2+b^2 \\ b^2=36-9 \\ b=\sqrt[]{27} \\ b=\sqrt[]{9\cdot3} \\ b=3\sqrt[]{3} \end{gathered}[/tex]