Question 23032
Its been a while since I've done this, so bear with me if the answer is kind of flawed. 


From what I interpreted of the question, I got that:

(AB U CD) at E (U means intersects)

AE = 6cm
BE = 8cm
CE = 3DE

Replace your unknowns with x.

DE = x
and CE = 3DE = 3x

Now, use the property that says....(I think, check behind me)

(AE)(EB) = (CE)(ED)
So we have,
6 * 8 = 3x * x
48 = 3x^2
x^2 = 16
x = +/- 4
(discard the negative answer, because distance cannot be negative.)

Now, substitute back;

ED = x = 4
CE = 3x = 3(4) = 12
So CD = CE + ED

CD = 16


There you go!