$\begin{array}{l}
Xet\,\,\Delta ABC\,\,\,va\,\,\,\Delta MNE\,\,\,co:\\
\angle A = \angle M = {70^0}\\
AB = EM\\
\Rightarrow \Delta ABC = \Delta MEN.\\
\Rightarrow \left\{ \begin{array}{l}
\angle C = \angle N = {50^0}\\
MN = AB = 3cm
\end{array} \right..\\
Lai\,\,\,co\,\,\,\angle A + \angle B + \angle C = {180^0}\\
\Rightarrow \angle B = {180^0} - \angle A - \angle C = {180^0} - {70^0} - {50^0} = {60^0}.\\
\Rightarrow \angle B = \angle E = {60^0}.
\end{array}$