(1).(x-2y)(x+2y)-(x+2y)^2

(1).(x-2y)(x+2y)-(x+2y)^2
=[x^2-(2y)^2]-(x^2+4xy+4y^2)
=x^2-4y^2-x^2-4xy-4y^2
=-4xy-8y^2
(2).(2a+1)^2-(1-2a)^2
=[(2a+1)+(1-2a)][(2a+1)-(1-2a)]
=(2a+1+1-2a)(2a+1-1+2a)
=2*4a
=8a
(3).(3x-y)^2-(2x+y)^2+5x(y-x)
=[(3x-y)+(2x+y)][(3x-y)-(2x+y)]+5x(y-x)
=(3x-y+2x+y)(3x-y-2x-y)+5x(y-x)
=5x*(x-2y)+5x(y-x)
=5x(x-2y+y-x)
=5x(-y)
=-5xy