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