Puting equation numbers on single isolated display formulas poses no special 
problen as plain \TeX\ provides two control sequences, {\tt \\eqno} for numbers
that are to go on the right, and {\tt \\leqno} for numbers that are to go on the 
left. In both cases, these control sequences go after the formula to be numbered.
they act much like our old friend `\&' in that they separate two fields,
everything to the left is part of the formula and
everything between them and the
terminating \$\$ will appear as the equation number. Thus,
\$\$(x+y)(x-y)=x\↑{}2-y\↑{}2.\\eqno(21).\$\$  will produce:
$$(x+y)(x-y)=x↑2-y↑2.\eqno(21)$$
If you type
\$\$(x+y)(x-y)=x\↑{}2-y\↑{}2.\\leqno[22].\$\$  you will get:
$$(x+y)(x-y)=x↑2-y↑2.\leqno[21a]$$
You will observe that the formulas are centered in both cases without
regard for the presence of the formula numbers. The numbers are in math style
unless you specify otherwise.

When several display formulas appear together, it is customary to align
them in some fashion.  \TeX\ provides a special control sequence for this
purpose, called {\tt \\eqalign}', which works with {\tt \&} and {\tt \\cr}
in a manner somewhat similiar to the use of these markers in {\tt \\matrix} and
{\tt \\cases}. For example, you type:

\$\$\\eqalign$\{$ax\↑{}2+bx+c\&=0\\cr
x\&=
$\{$-b\\pm
\\sqrt$\{$b\↑{}2-4ac$\}$\\over2a$\}$.\\cr$\}$\$\$

to get
$$\eqalign{ax↑2+bx+c&=0\cr
x&=
{-b\pm
\sqrt{b↑2-4ac}\over2a}.\cr}$$

\$\$\\eqalign$\{$ax\↑{}2+bx+c\&=0\\cr
x\&=
$\{$-b\\pm
\\sqrt$\{$b\↑{}2-4ac$\}$\\over2a$\}$.\\cr$\}\\eqno(2)$\$\$

to get
$$\eqalign{ax↑2+bx+c&=0\cr
x&=
{-b\pm
\sqrt{b↑2-4ac}\over2a}.\cr}\eqno(3)$$

\$\$\\eqalignno$\{$ax\↑{}2+bx+c\&=0\&(1)\\cr
x\&=
$\{$-b\\pm
\\sqrt$\{$b\↑{}2-4ac$\}$\\over2a$\}$.\&(3)\\cr$\}$\$\$

to get
$$\eqalignno{ax↑2+bx+c&=0&(1)\cr
x&=
{-b\pm
\sqrt{b↑2-4ac}\over2a}.&(2)\cr}$$

\$\$\\leqalignno$\{$ax\↑{}2+bx+c\&=0\&(1)\\cr
x\&=
$\{$-b\\pm
\\sqrt$\{$b\↑{}2-4ac$\}$\\over2a$\}$.\&(3)\\cr$\}$\$\$

to get
$$\leqalignno{ax↑2+bx+c&=0&(1)\cr
x&=
{-b\pm
\sqrt{b↑2-4ac}\over2a}.&(2)\cr}$$

to get
$$\leqalignno{ax↑2+bx+c&=0&{\rm given\ that}\cr
x&=
{-b\pm
\sqrt{b↑2-4ac}\over2a}.&{\rm then}\cr}$$

\bye