This problem is about a right triangle, which means we have to use the Pythagorean Theorem to find the missing leg.

Always keep in mind that c represents the hypotenuse (the longest side), while a and b are the legs. In this case, we have c = 21 and a = 14. Let's use these values and find b.

Now, we solve for b.
![\begin{gathered} 441=196+b^2 \\ 441-196=b^2 \\ 245=b^2 \\ b=\sqrt[]{245} \\ b\approx15.7 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/v34qe9wvjrasykmoma4exdrthw2rr583mk.png)
Therefore, the missing leg is 15.7 units long.