x^2*y^2 part giv a positiv number that kan get very big if x or y get big
the other part...(4-x^2)*(4-y^2) is same as [-(x^2-4)]*[-(y^2-4)]
2 minus sines kansel, so it become (x^2-4)*(y^2-4)
& that be...(x-2)(x+2)(y-2)(y+2)
if x or y get big, that thang kan get plenty big...big as yu want