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