what is the length of EF in the right triangle below?

Use the Pythagorean theorem:
[tex]|DF|^2+|EF|^2=|DE|^2[/tex]
We have |DF| = 12 and |DE| = 19. Substitute:
[tex]12^2+|EF|^2=19^2\\\\144+|EF|^2=361\qquad|-144\\\\|EF|^2=217\to|EF|=\sqrt{217}[/tex]
Answer: [tex]C.\ \sqrt{217}[/tex]