nedd help

Note that $\frac{dy}{dx}(log_a x)=\frac{1}{x\ln a}$. So $\frac{dy}{dr}(log_2 r * log_4 r)=\frac{dy}{dr}(log_2 r)*\frac{dy}{dr}(log_4 r)=\frac{1}{r\ln 2}*\frac{1}{r\ln 4}=\frac{1}{r^2\ln 2\ln 4}$