How to write this matrix equation in latex in the center position?

Question

I want to write something like in the images by using

I also want to place a subscript in the right bottom part of the very right matrices. Thanks in advance.

Answer 1

You should seek an introductory book on LaTeX and how to do things in math mode, in particular. See What are good learning resources for a LaTeX beginner?.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
\begin{pmatrix}
m\\k
\end{pmatrix}
= X
\begin{pmatrix}
t\\u
\end{pmatrix}
_r
\]
\end{document}

It is possible, depending on your notation, that you would prefer the look of \mathbf{X} in lieu of X.

And Bernard rightly suggests that the subscript could be tucked in closer to the vector, by employing the subscript as _{\mkern-5mu r} rather than simply as _r.

By popular demand, the alternative with stackengine:

\documentclass{article}
\usepackage{stackengine}
\begin{document}
\[ \parenVectorstack{m k}= X \parenVectorstack{t u}_r \]
\end{document}