Answer:
5
Explanation:
Since the triangles are similar
The ratios have to be the same
AC BC
---------- = -------------
CE CD
21 20-x
--------- = -----------
7 x
Using cross products
21x = 7(20-x)
Distribute
21x = 140-7x
Add 7x to each side
21x+7x = 140-7x+7x
28x = 140
Divide each side by 28
28x/28 = 140/28
x =5
We want the length of CD
CD = x= 5