Answer:
36
Explanation:
use the Pythagorean theorem, which is a^2+b^2=c^2 where c is always the hypotenuse and a & b can be interchangeable because they are just the legs.
in this case the hyp is 39
so 39^2=15^2+EF^2
which can also be 39^2-15^2=ef^2
1521-225=ef^2
now square root both sides to get just ef
ef= square root of 1296
ef=36