What is the way to achieve something between gather and aligned?
\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage[utf8]{inputenc}
\begin{document}
\begin{gather}
3(a-x) = 3.5x + a - 1 \
3a - 3x = 3.5x + a -1 \
a = \frac{13}{4}x - \frac{1}{2}
\end{gather}
\begin{equation}
\begin{aligned}
3(a-x) = 3.5x + a - 1 \
3a - 3x = 3.5x + a -1 \
a = \frac{13}{4}x - \frac{1}{2}
\end{aligned}
\end{equation}
\end{document}
Result:
But...
How to achieve something like this? I'd like to keep the centering.

Is this possible somehow?


\kern-50ptbefore\begin{gathered}Here I used an arbitrary/context dependent negative left shift of length 50pt. Specifically, you can use any length to suit your need. Warning: most would advise against this because the math will overlap the left margin. – Alberto Takase Feb 23 '21 at 23:14alignedinstead ofgathered. Just swap the terms and add&s to the=s. – Suuuehgi Aug 08 '22 at 16:01