When equation
starts, TeX is in math mode, so no $
is necessary inside the environment; to be more precise, a $
inside the environment will cause errors.
By the way, even in line you should not split formulas in that way; the input
$x$ - $y$
in normal text will produce a hyphen and not a minus sign, which is probably what you want.
Also ^{_{A}}
is wrong: the exponent is reduced in size, but positioned wrongly. If you really want a smaller exponent, the right way is
^{\scriptscriptstyle A}
but I strongly discourage you to do it.
You also have a useless aligned
environment, that would only be useful for splitting the equation across lines. Note also that \bar
is better than \overline
in this case.
Correct input:
\begin{equation}\label{3-5}
u^{A}_{i} - u^{B}_{i} = \bar{\epsilon}_{ik}x^{A}_{k} -\bar{\epsilon}_{ik}x^{B}_{k}
\end{equation}
You may want to fine tune the positioning of subscripts by adding dummy superscripts to \bar{\epsilon}
:
\begin{equation}\label{3-5}
u^{A}_{i} - u^{B}_{i} = \bar{\epsilon}^{}_{ik}x^{A}_{k} -\bar{\epsilon}^{}_{ik}x^{B}_{k}
\end{equation}
$
insideequation
. By the way,aligned
does exactly nothing, here.