How to use tabs in Latex?

Welcome! You can use the aligned environment with [t] for that.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
Set of equations: $\begin{aligned}[t]x &= y + 5\\
                  y &= 3\end{aligned}$
\end{document}

You can use the standard tabbing environment for this.

\documentclass{article}
\begin{document}
\begin{tabbing}
  Set of equations: \= $x = y + 5$ \\
                    \> $y = 3$     \\
\end{tabbing}
\end{document}

I would do it with flalign(*):

\usepackage{amsmath}
.........
\begin{flalign*}
&\rlap{Set of equations:} & x &= y + 5 & \\
              & &   y &= 3
\end{flalign*}