Both lines AB and CD are secant lines beacuse in two points they are touching the circunference. There is a Theorem which says the following
Since the distance from the center of the circle to each secant line is the same (5 units), we could assume that the both secant lines are similar. saying:
Then the lenght of CD is:
So we can factor out th 2 in both becuaase of distributeive porperty
ab+ac=a(b+c)
so
2l+2w=2(l+w)
If your adding then it will be 6