Assuming that the figure referred to here is a straight line, then this means that point C, point D, and point E are all collinear. With points C and E located at the ends while point D is located in between point C and point E, such that:
C -------- D ------- E
With the given figure above, we can say the following expression:
CD + DE = CE
Since we are to find for CD, we rewrite the equation in terms of CD by transposing DE to the right side:
CD = CE – DE
Substituting the values:
CD = 17.1 – 8
CD = 9.1