This problem can be concluded as **colorbox inside align**, so we can use
```tex
\documentclass{article}
\usepackage{xcolor}
\usepackage{mathtools}

% Reference: https://tex.stackexchange.com/a/13693/234654
\newlength\dlf
\newcommand\alignedbox[2]{
 % #1 = before alignment
 % #2 = after alignment
 &
 \begingroup
 \settowidth\dlf{$\displaystyle #1$}
 \addtolength\dlf{\fboxsep+\fboxrule}
 \hspace{-\dlf}
 \fcolorbox{yellow}{yellow}{$\displaystyle #1 #2$}
 \endgroup
}

\begin{document}
\begin{align*}
 %--
 i^7 &= 1 \times i^3 \\[5mm]
 %--
 i^7 &= i^3 \\[5mm]
 %--
 i^7 &= i^2 \times i \\[5mm]
 %--
 i^7 &= -1 \times i \\[5mm]
 %--
 i^7 &= -i \\[5mm]
 %--
 \alignedbox{i^7}{=-i}
\end{align*}
\end{document}
```
This genius idea is borrowed from [here](https://tex.stackexchange.com/a/13693/234654).

This method calculates the length before the aligned mark `&` into variable `\dlf`. Then `\hspace` is used to left shift the colorbox. Now the alignment is achieved.

[![RESULT][1]][1]


 [1]: https://i.sstatic.net/c11YL.png