a: \(=\dfrac{4\left(m-n\right)^2}{5\left(m-n\right)\left(m+n\right)}=\dfrac{4m-4n}{5m+5n}\)
b: \(=\dfrac{x\left(x-y\right)-z\left(x-y\right)}{x\left(x+y\right)-z\left(x+y\right)}=\dfrac{x-y}{x+y}\)
c: \(=\dfrac{y\left(x-3\right)-9\left(x-3\right)}{-\left(x-3\right)}=-y+9\)


