\(a,=\left(x+y\right)^2-z\left(x+y\right)=\left(x+y-z\right)\left(x+y\right)\\ =2020\left(2020-2019\right)=2020\cdot1=2020\\ b,=\left(x+y\right)^3+\left(x+y\right)=\left(x+y\right)\left[\left(x+y\right)^2+1\right]\\ =10\cdot\left(10^2+1\right)=10\cdot101=1010\)


