lign*} 
a_1& =b_1+c_1\\ 
a_2& =b_2+c_2-d_2+e_2 
\end{align*}

Align:

\begin{align} 
a_{11}& =b_{11}& a_{12}& =b_{12}\\ 
a_{21}& =b_{21}& a_{22}& =b_{22}+c_{22} 
\end{align}

Align with \notag and \tag:

\begin{align} 
a_{11}& =b_{11}& a_{12}& =b_{12}\notag\\ 
a_{21}& =b_{21}& a_{22}& =b_{22}+c_{22} \tag{y}
\end{align}

Align* with \tag:

\begin{align*} 
a_1& =b_1+c_1\tag{z}\\ 
a_2& =b_2+c_2-d_2+e_2 
\end{align*}

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