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What is the radius of a circle tangent to the lines y = 3x + 7 and y = .5x - 3 and containing the point (8,1)?

I've determined that the given point (8,1) is the point of tangency of the line y = .5x - 3 and the circle. Also, the tangent lines intersect at (-4,-5) and the distance between (-4,-5) and (8,1) is approximately 13.416. It seems like I would need to find the center of the circle and use the Pythagorean Theorem to find the radius, but I can't figure out how to find the center. The solution is 5.56. Any suggestions?