How to make an equation for a circle?

The equation is x^2+y^2=R^2 . so you use the distance formula to find the radius. -4-8=-12 -12^2= 144.... -3--1=-2 -2^2=4 144+4=148 then you find the square root of 148 which is 12.1655 then to rind the radius you divide it by two and get 6.08=R