Let's take a look at our triangle:
Using the pythagorean theorem, we'll have that:
[tex]h^2=12^2+16^2[/tex]Solving for h,
[tex]\begin{gathered} h^2=12^2+16^2 \\ \rightarrow h=\sqrt[]{12^2+16^2} \\ \rightarrow h=\sqrt[]{400} \\ \\ \Rightarrow h=20 \end{gathered}[/tex]This way, we can conlcude that the missing side measures 20 units.