There is only 1 solution for the length of BC.
We can calculate it using Pythagorean theorem.
We can conclude that triangle's hypotenuse is AC.
[tex]AC^2=AB^2+BC^2[/tex]
If that is the case we are looking for a side BC.
[tex]BC=\sqrt{AC^2-AB^2}[/tex]
Now put in the data.
[tex]BC=\sqrt{21^2-13^2}=\sqrt{272}=\sqrt{272}\approx\boxed{16.49}[/tex]
Hope this helps.
r3t40