b. \(x^2-2xy-4+y^2\)
= \(\left(x-y\right)^2-4\)
= \(\left(x-y-2\right)\left(x-y+2\right)\)
c. \(x^2-y^2+2019x+2019y\)
= \(\left(x-y\right)\left(x+y\right)+2019\left(x+y\right)\)
=\(\left(x+y\right)\left(x-y+2019\right)\)
f. \(x^2-5x+6\)
= \(x^2-3x-2x+6\)
= \(x\left(x-3\right)-2\left(x-3\right)\)
= \(\left(x-2\right)\left(x-3\right)\)