In the following example, how can I shift the \Rightarrow slightly towards left so that the first terms of both equations are vertically aligned?
\documentclass[10pt]{article}
\usepackage{mathtools}
\usepackage{mathtools}
\usepackage{amsmath}
\usepackage{amsthm} %for proof
\begin{document}
\begin{align*}
\frac{\partial V(u)}{\partial b}&=2.b.\sigma_{X}^{2}-2.\sigma_{XY}=0 \\
\Rightarrow b &=\frac{\sigma_{XY}}{\sigma_{X}^{2}} = \beta^{*}
\end{align*}
\end{document}

\quadbetween\Rightarrowandb. – egreg Sep 02 '14 at 17:44