1) (2a-3b)(2a+3b)-4a^2-9b^2
4a^2-3a^2=1a^2
2)(3y+4z)^2=9y^2+24yz+16z^2
8(3y-2z)=24y-16z
(24y-16z)z=24yz-16z^2
9y^2+24yz+16z^2-(24yz-16z^2)=9y^2+24yz+16z^2-24yz+16z^2
24yz-24yz=0
16z^2+16z^2=32z^2
3)(3x-4y)^2=9x^2-24xy+16y^2
(2x-7y)(4x+2y)=8x^2-24xy-14y^2
9x^2-24xy+16y^2-(8x^2-24xy-14^2)=9x^2-24xy+16y^2-8x^2+24xy+14y^2
9x^2-8x^2=1x^2
-24xy+24xy=0
16y^2+14y^2=30y^2