This problem is about a right triangle, which means we have to use the Pythagorean Theorem to find the missing leg.
[tex]c^2=a^2+b^2[/tex]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.
[tex]21^2=14^2+b^2[/tex]Now, we solve for b.
[tex]\begin{gathered} 441=196+b^2 \\ 441-196=b^2 \\ 245=b^2 \\ b=\sqrt[]{245} \\ b\approx15.7 \end{gathered}[/tex]