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:
[tex]|AB|=\sqrt{(x_2-x_1)^2+(y_2-y_1)^2} \\ \\ \hbox{So:} \\ \\ r=|SA|=\sqrt{(-3-10)^2+(3-6)^2}=\sqrt{(-13)^2+(-3)^2}=\sqrt{169+9}= \\ r= \sqrt{178}[/tex]
