a) `-7x^2yz-3/7 x^2yz = (-7-3/7) x^2yz=-52/7 x^2yz`
b) `1/4 cy^2 -1/2xy^2+(-4/5xy^2)=(1/4-1/2-4/5)xy^2=-21/20 xy^2`
c) `xyz-4xyz+5xyz+6xyz=(1-4+5+6)xyz=8xyz`
d) `-1/2(x^2y^2)^2-5x^4y^4+6x^4y^4+23x^4y^4`
`=-1/2 x^4y^4 -5x^4y^4+6x^4y^4+23x^4y^4`
`=(-1/2 -5+6+23)x^4y^4`
`=47/2 x^4y^4`