we are on problem 61 It says the point minus 3, 2 lies on a circle whose equation is x plus 3 squared plus y plus 1 squared is equal to r squared. Which of the following must be the radius of the circle? So the way to think about it is, is that this point satisfies this equation. Any point on the equation will satisfy both sides of this equality sign. So all we have to do is substitute the x and the y here and see what r squared has to be equal to. here and see what r squared has to be equal to. If we substitute the minus 3 in for x. You get minus 3, I just substituted for the x. Plus 3 squared plus, now y, y is 2. 2 plus 1 squared is equal to r squared.