x^2+ 3x+ ?= (x+ a)^2
y^2+ y+ ?= (y+b)^2
The two "?"s will be numbers of course and you add the same numbers to the right side of the equation. The result will be something like (x+a)^2+ (y+b)^2= r^2; the graph is a circle with center at (-a, -b) and radius a.