Try to use the fleqn document class option.
documentclass[fleqn]{article}
(See also http://en.wikibooks.org/wiki/LaTeX/Basics for a list of other options.)
You can use begin{flalign}, like the example bellow:
begin{flalign}
&f(x) = -1.25x^{2} + 1.5x&
end{flalign}