\(\left(2x-y\right)^3-8-3\left(x-y\right)\left(2x-y-2\right)\\ =\left(2x-y-2\right)\left[\left(2x-y\right)^2+2\left(2x-y\right)+2^2\right]-\left(3x-3y\right)\left(2x-y-2\right)\\ =\left(2x-y-2\right)\left(4x^2-4xy+y^2+4x-2y+4-3x+3y\right)\\ =\left(2x-y-2\right)\left(4x^2-4xy+y^2+x+y+4\right)\)