\documentclass{jarticle}
\usepackage{amsmath}
\newcommand{\hako}[1]{\ \fbox{\ \textbf #1\ }\ }
\begin{document}
\begin{equation}\label{eq:eq1}
\begin{array}{ccc}
\text{(a)}&a_{n+2}&=\hako{オ}a_{n+1}+\hako{カ}a_n\tag{$n=1,2,3,\cdots$}\\[5pt]
\text{(b)}&b_{n+2}&=\hako{キ}b_{n+1}+\hako{ク}b_n
\end{array}
\end{equation}
\end{document}
