a: \(5x^2\left(x-2y\right)-15xy\left(x-2y\right)\)
\(=\left(x-2y\right)\left(5x^2-15xy\right)\)
\(=\left(5x\cdot x-5x\cdot3y\right)\left(x-2y\right)\)
\(=5x\left(x-3y\right)\left(x-2y\right)\)
b: \(x\left(x+y\right)+4x+4y\)
\(=x\left(x+y\right)+\left(4x+4y\right)\)
\(=x\left(x+y\right)+4\left(x+y\right)\)
\(=\left(x+y\right)\left(x+4\right)\)