\(a,-7x^2yz-\dfrac{3}{7}x^2yz=-\dfrac{52}{7}x^2yz.\)
\(b,\dfrac{1}{4}xy^2-\dfrac{1}{2}xy^2+\left(-\dfrac{4}{5}xy^2\right)=-\dfrac{1}{4}xy^2-\dfrac{4}{5}xy^2=\dfrac{-21}{20}xy^2.\)
\(c,xyz-4xyz+5xyz-6xyz=-4xyz.\)
\(d,\left(-\dfrac{1}{2}x^2y^2\right)^2-5x^4y^4-6x^4y^4-23x^4y^4=\dfrac{1}{4}x^4y^4-34x^4y^4=-33,75x^4y^4.\)