The radius is 8
Im not sure if this is what your looking for
Answer:
The solution is 
Step-by-step explanation:
We have the following equation:

Elevating both sides to 2, we can isolate x




Checking

You just subtract 4 from 20
20-4 which is 16
It is 19. because if it’s -8 you add the other 8 so that makes 0 and then there’s 19 left so it’s 19