10y-10z-(y-z)^2
10y- 10z -(y^2-2yz+z^2)=10y-10z-y^2+2yz-z^2
10y-y^2-2yz=y(10-y-2z)
10 z - z^2 =z(10-z)
y(10-y) -z(10-z) + 2yz =(y-z)(z-y+10)
