$N=\frac{15d}{4}\text{ and }\frac{15d}{4}=42cm$

$\left\{\begin{matrix}m=\frac{5d}{56c}\text{, }&N=\frac{15d}{4}\text{ and }c\neq 0\\m\in \mathrm{R}\text{, }&N=0\text{ and }d=0\text{ and }c=0\end{matrix}\right.$

$d=\frac{56cm}{5}$
$N=42cm$