\(a,2x^2+4x=2x\left(x+2\right)\)
\(b,2\left(x-y\right)+5x\left(y-x\right)=2\left(x-y\right)+5x\left(-x+y\right)=2\left(x-y\right)-5x\left(x-y\right)=\left(x-y\right)\left(2-5x\right)\)
\(c,x^2+y^2-2xy-4=\left(x^2-2xy+y^2\right)-4=\left(x-y\right)^2-4=\left[\left(x-y\right)-2\right]\left[\left(x-y\right)+2\right]=\left(x-y-2\right)\left(x-y+2\right)\)


