a) \(6x\left(x-y\right)-3\left(x-y\right)^2\)
\(=3\left(x-y\right)\left(2x-x+y\right)\)
b) \(2a\left(x-1\right)+b\left(x-1\right)-\left(1-x\right)\)
\(=2a\left(x-1\right)+b\left(x-1\right)+\left(x-1\right)\)
\(=\left(x-1\right)\left(2a+b+1\right)\)
c) \(2x\left(x-2\right)-\left(x-2\right)^2+5x\left(2-x\right)\)
\(=2x\left(x-2\right)-\left(x-2\right)^2-5x\left(x-2\right)\)
\(=\left(x-2\right)\left(2x-x+2-5x\right)\)
\(=\left(x-2\right)\left(-4x+2\right)\)