Actually Welcome to the concept of Parallel lines.
We must first understand that, Parallel Lines always have a same Slope, hence the 'm' value in y=mx+c equation will same, here it is '1/2' in the above equation,
The radius is from center to point on a circle. So your task is count distance between two points: S(10,6) and A(-3,3). Distance between two points A(x1, y1) and B(x2, y2) count from this formula: