a: \(=x\left(3x-y\right)+4\left(3x-y\right)=\left(3x-y\right)\left(x+4\right)\)
b: \(=5x\left(x-3y\right)+4x\left(x-3y\right)=9x\left(x-3y\right)\)
f: \(=-\left(x-2y\right)\left(x+2y\right)+\left(x-2y\right)\)
\(=\left(x-2y\right)\left(-x-2y+1\right)\)