We have a right triangle, for which we know the two legs.
We can calculate the hypotenuse H by applying the Pythagorean theorem:
[tex]\begin{gathered} H^2=L^2_1+L^2_2=8^2+21^2=64+441=505 \\ H=\sqrt[]{505}\approx22.5 \end{gathered}[/tex]The missing length (the hypotenuse) is approximately 22.5 cm.
