1)16x^2 -12xy-9y^2 +12xy + 12xy+16y^2 -12xy-9x^2=16x^2 -9y^2 + 16y^2 --9x^2=7x^2+7y^2
2)x^2 +4x+4 - (x^2 +3x-3x-9)=x^2+4x+4-x^2+9=4x+13
3)49a^2+35ab-35ab-25b^2 -(16a^2 +56ab+49b^2)=49a^2-25b^2 -16a^2 -56ab-49b^2=33a^2 - 74b^2 -56ab
4)y^2 -2y-6+3y-y^2 +2y -1+5y-y^2-5y+25= -6+3y -1-y^2+25=-y^2+3y+18