Consider this option:
1. rule: Circle is the locus of the points which are on the same distance from the given point. Common view of the equation is: (x-a)²+(y-b)²=r², where r - the distance from the given point to the described locus, a&b - coordinates of the given point.
2. using item 1:
(x+2)²+(y-1)²=3² or (x+2)²+(y-1)²=9.