501

a)
D\left(\frac{1}{x^2}\right)=x^{-2}
b)
D\left(\frac{4}{x^3}\right)=4\cdot\frac{1}{x^3}=4x^{-3}=12x^{-4}=\frac{12}{x^4}
c)
D\left(-\frac{6}{x^7}\right)=-6x^{-7}=-42x^{-8}=-\frac{42}{x^{-8}}