\eqalign - Used for equation alignment; for aligning multi-line displays at a single place.
{ \eqalign{ <math> & <math> \cr <repeat as needed> } }
\eqalign 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.
\eqalign{ 3x - 4y &= 5\cr x + 7 &= -2y }
\eqalign{ (a+b)^2 &= (a+b)(a+b) \\ &= a^2 + ab + ba + b^2 \\ &= a^2 + 2ab + b^2 }
\left\{ \eqalign{ a &= 1\\ b &= 2\\ c &= 3 }\right\} \qquad \eqalign{ ax + by &= c \\ x + 2y &= 3 }