The radius of circle O is the same as half of the hypotenuse of the right triangle.
The find the hypotenuse, simply use the following,

Or the square root of one leg squared us the other leg squared.
I already know this to be a 5 - 12 - 13 right triangle and you can get 13 with the equation.
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Half of 13 is 6.5 meaning 6.5 is our radius.
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I hope that helps you out!!
Any more questions, please feel free to ask me and I will gladly help you out!!
~Zoey