To find a missing side in a right triangle you use the Pythagoras theorem:
[tex]h^2=(l_1)^2+(l_2)^2[/tex]
You have:
h=10
l1=9
[tex]\begin{gathered} 10^2=9^2+(l_2)^2 \\ 100=81+(l_2)^2 \\ 100-81=(l_2)^2 \\ 19=(l_2)^2 \\ \sqrt[]{19}=l_2 \\ \end{gathered}[/tex]Then, the missig side length is square root of 19
