\eqalignno - Used for equation alignment with optionally numbered (tagged) lines.
{ \eqalignno{ <math> & <math> & <equation tag> \cr <repeat as needed> } }
\eqalignno command is used for equation alignment; for aligning multi-line displays at a single place. The ampersand is placed where alignment is desired; a double-backslash can be used in place of the \cr ; the final \\ or \cr is optional; supports only a single \tag, which is vertically centered. if there is no tag, then the final & <equation tag> is omitted;
\eqalignno{ 3x - 4y &= 5 &(\dagger) \cr x + 7 &= -2y &(\ddagger)\cr z &= 2 }