Answer:
6
Explanation:
imagine that EF continues through F and ends on AB. this point, that is on segment AB, is what i will call G.
AFG is similar to CFE.
the length of segment GF is 18-4=14.
now we know 2 side lengths of triangle GAF: FA is 21 and FG is 14.
FE is similar to GF with a factor of 4/14 = 2/7.
FE is similar to FC with this same ratio: (2/7) * 21 = 6
therefore, the length of FC is 6.