Question

The point (-3,3) lies on a circle. What is the length of the radius of this circle if the center is located at (10,6) ?

Answer 1

 You should find distance between two points 
A(x1, y1) and B(x2, y2)

Answer 2

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