Question

what is the measure of the secant DC ?

Answer 1

For secants intersecting outside a circle, you have:
(CB)*(CA) = (CD)*(CE)

Answer 2

should i multiply CB and CA?

Answer 3

Yes, since you know their lengths.

Answer 4

and CD and CE right ?

Answer 5

26= CD*14

Answer 6

Be careful, CB = 4, what is CA?

Answer 7

6.5?

Answer 8

BA = 6.5
CA = CB + BA

Answer 9

What is CA?

Answer 10

10.5

Answer 11

Now you got it.
Now do CB * CA

Answer 12

42

Answer 13

CB * CA = CD * CE
4 * 10.5 = CD * 14
42 = CD * 14
Now solve for CD

Answer 14

Yes, 42 is right.

Answer 15

Cd= 28 ?

Answer 16

42 = CD * 14
CD = 42/14
42/14 is not equal to 28

Answer 17

then its 3

Answer 18

That's much better. That's it.
Just remember that when you have two secants that intersect outside a circle, the product of one secant and its exterior segment equals the product of the other secant and its extrior segment.