#### Answer

$f=10sin(36)$

#### Work Step by Step

Since this is a pentagon, the angle is given by:
360/10=36
If we draw a triangle extending to the center point between each corner, we find that the length of this side $f/2$ is:
$sin(36)= (f/2)/a$
This gives:
$2asin(36)=f$
Since a=5:
$f=10sin(36)$