Left-aligning equations

How do I left-align selective equation blocks, i.e. not centered ?

I know how to do it for ALL equation blocks in the document setting

\usepackage[fleqn]{amsmath}
\setlength{\mathindent}{0pt}


in the preamble, but I only want to do this for some equation blocks.

\documentclass[pdftex,12pt,a4paper,english,dutch,leqno]{article}
\usepackage{babel}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage[dvipsnames,table]{xcolor}
\usepackage{color}
\definecolor{vanillegeel}{RGB}{255,240,216}
\usepackage{empheq}

\begin{document}

De raaklijn aan $(x_{0},f(x_{0}))$ wordt algemeen bepaald met :
\begin{empheq}[box={\fboxsep=3pt\colorbox{vanillegeel}}]{align*}
\textnormal{$\begin{cases}z=f(x)\text{ is continu in }x_{0}\text{.}\\ f'(x_{0})=0\text{ : }z=f(x_{0})\text{ is de horizontale raaklijn.}\\ \lim_{x\to x_{0}} f'(x)=\pm\infty\text{ : }x=x_{0}\text{ is de verticale raaklijn.}\\ f'(x_{0})\in\mathbb{R}_{0}\text{ : }z-f(x_{0})=f'(x_{0})\cdot(x-x_{0})\text{ is de raaklijn.\hspace{-12pt}}\end{cases}$}\end{empheq}

\end{document}

-
+1 for the background color! Really nice for the eyes. – morbusg Oct 5 '12 at 13:30
Place '&' at the beginning of each line inside the empheq environment and do not use the cases environment. This is the easiest way to get what you want. – bobb_the_builder Jul 17 at 16:23

Don't use align*.

De raaklijn aan $(x_{0},f(x_{0}))$ wordt algemeen bepaald met:
\begin{flushleft}
\colorbox{vanillegeel}{%
$\displaystyle \begin{cases} z=f(x) \text{ is continu in } x_{0}. \\ f'(x_{0})=0 \text{ : } z=f(x_{0}) \text{ is de horizontale raaklijn.} \\ \lim_{x\to x_{0}} f'(x)=\pm\infty \text{ : } x=x_{0} \text{ is de verticale raaklijn.} \\ f'(x_{0})\in\mathbb{R}_{0} \text{ : } z-f(x_{0})=f'(x_{0})\cdot(x-x_{0}) \text{ is de raaklijn.} \end{cases}$\kern-\nulldelimiterspace\kern-2\arraycolsep}
\end{flushleft}


-
What does \kern-\nulldelimiterspace\kern-2\arraycolsep} do in this ? – Petoetje59 Oct 5 '12 at 14:28
@Petoetje59 It's a substitute of your \hspace{-12pt} based on document parameters, rather than "calculation by eye". – egreg Oct 5 '12 at 14:45
Not allowing you to get the unsung hero badge! ;) +1. – Harish Kumar Oct 5 '12 at 14:48
@HarishKumar Darn! – egreg Oct 5 '12 at 14:49
How come \begin{align*} doesn't work but flushleft does? – White House Fence Jumper Sep 1 at 8:18