Chapter 4.2, Problem 51E

Show that the integer m with two’s complement representation $\left(a_{n-\text{1}}{a}_{n-\text{2}\mathrm{..}}{a}_{\text{1}}{a}_0\right)$ can be found using the equation $m=-a_{n-\text{1}}\cdot {\text{2}}^{n-\text{1}}{}_{+}{a}_{n-\text{2}}{2}^{n-\text{2}}+\mathrm{...}+a_{\text{1}}.\text{ 2}+a_{\text{0}}$_.