Answer: 13 units
Step-by-step explanation:Given that Point C is the midpoint of AB and point B is between points A and D. If AD = 17 and BD = 9
According to definition midpoint, AC = CB
According to definition of segment addition postulate, AD = AB + BD
AB = AD - BD
AB = 17 - 9
AB = 8
Again, AB = AC + CB
Therefore, CB = AB/2
CB = 8/2 = 4
Now, CD = CB + BD = 4 + 9
CD = 13 units
Hence, length of CD = 13 units