Hi there!
Let's use the relationships we know with the external secants. This would give us the equation: 8(20) + 10(10 +x)
WORK:
160 = 100 + 10x
60 = 10x
x = 6
Add 10 and 6 together
ANSWER:
AD = 16
Hope this helps!! :)
If there's anything else that I can help you with, please let me know!