6x-6y-9x^2+18xy-9y^2-1
=-9(x^2-2xy+y^2)+6(x-y)-1
=-9(x-y)^2+6(x-y)-1
=-[3(x-y)]^2+6(x-y)-1
=-{[3(x-y)]^2-2*[3(x-y)]*1+1^2}
=-[3(x-y)-1]^2