a: =(x-y)(x+y)-2(x-y)=(x-y)(x+y-2)
b: \(=2\left(x+y\right)-x\left(x+y\right)=\left(x+y\right)\left(2-x\right)\)
c: \(=3\left(a^2-2ab+b^2-4c^2\right)\)
\(=3\left(a-b-2c\right)\left(a-b+2c\right)\)
d: \(=\left(x+y\right)^2-25=\left(x+y+5\right)\left(x+y-5\right)\)