We draw those points:


(1,-7) , (-1,-9), and (3,-9) and center (1,-9)
We can only draw this circle with that center and going through
those points:



So draw in three radii:



Look at the red radius. it goes from -7 down to -9, that's 2 units
Look at the green radius, it's also 2 units long, and the blue one
is also 2.  So the radius is 2. 

Edwin