# Aligning equations in \begin{cases}

I need neither of the following:

$$\begin{cases} ab =st \\ a-b =s+t \end{cases}$$

$$\begin{cases} ab &=st \\ a-b &=s+t \end{cases}$$


Instead, I want something like this with bracket on the left:

 ab=st
a-b=s+t


How to achieve this? Thanks.

\documentclass{article}
$$\left\{ \begin{array}{@{} r @{{}={}} l} ab &st\\ a-b &s+t \end{array} \right.$$

• If you want to emulate cases, add \renewcommand{\arraystretch}{1.2} after \left\{. – egreg May 12 '18 at 6:31
• if loading mathtools, it's redundant to load amsmath`. – barbara beeton May 13 '18 at 22:05