What is the length of ? 3 ft 4 ft 9 ft 18 ft

AC = 18 ft

Explanation:
There is a geometric theorem that states that:
" In a triangle, the line joining two midpoints of two sides is parallel to the third side and equals have its length"

Now, let's check the given triangle:
M is the midpoint of AB
N is the midpoint of BC

Based on the above theorem:
MN will be parallel to AC
and
length of MN = (1/2) * length of AC

We are given that:
length of MN = 9 ft
Therefore:
9 = (1/2) * length of AC
length of AC = 2 * 9 = 18 ft

Hope this jhelps :)