\(5y\left(y-x\right)+15x\left(x-y\right)\)
= \(5y\left(y-x\right)-15x\left(-x+y\right)\)
= \(5y\left(y-x\right)-15x\left(y-x\right)\)
= \(\left(y-x\right)\cdot\left(5y-15x\right)\)
=\(\left(y-x\right)\cdot5\left(y-3x\right)\)
\(5y\left(y-x\right)+15x\left(x-y\right)\)
\(=-5y\left(x-y\right)+15x\left(x-y\right)\)
\(=5\left(x-y\right)\left(3x-y\right)\)