xy=27, x±y=21;
x=21∓y, so substitute for x: y(21∓y)=27,
21y∓y2=27⇒
y2+21y-27=0 or y2-21y+27=0.
y=(-21±√(441+108))/2 or y=(21±√(441-108))/2,
y=(-21±√549)/2 or y=(21±√333)/2.
y=1.2154 or -22.2154, x=22.2154 or -1.2154; or 19.6241 or 1.3759, x=1.3759 or 19.6241.