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\documentclass{article}
\usepackage[a4paper, total={6in, 8in}]{geometry}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{fourier}
\usepackage{mathtools}
\usepackage{amsmath}
\usepackage[english]{babel}
\usepackage{graphicx}
\usepackage{marginnote} 
\begin{document}
$$ KP = e^{-\lambda} \Big( \frac {\nu'}{r} +\frac {1}{r}\Big) -\frac {1}{r^2}$$
    \begin {gather}
    \Rightarrow \frac {dP}{dr} = - \frac { P+\rho (P) c^2}{r(r-2u)} \Big[ \frac {K}{2} P r^3 +u \Big] \tag {**}
    \end {gather}
\end {document}

How do I align them ?

Tazkera Haque Trina
  • 335
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    Please read http://tex.stackexchange.com/questions/503/why-is-preferable-to on the $$...$$ topic –  Aug 07 '14 at 06:29
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    Please help us to help you and add a minimal working example (MWE) that illustrates your problem. It will be much easier for us to reproduce your situation and find out what the issue is when we see compilable code, starting with \documentclass{...} and ending with \end{document}. –  Aug 07 '14 at 06:29
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    Put both of them in an align environment and use ampersands & to mark positions. – Johannes_B Aug 07 '14 at 06:40
  • Aligning equations is pretty basic. Please have a look at mathmode.pdf which you can find on CTAN. Section 25 has all the information you need. – Johannes_B Aug 07 '14 at 07:10

1 Answers1

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\documentclass{article}
\usepackage{mathtools}
\begin{document}
\begin{align*}
  KP &= e^{-\lambda} \Big( \frac {\nu'}{r} +\frac {1}{r}\Big) -\frac {1}{r^2} \\[2ex]
 \Rightarrow \frac {dP}{dr} &= - \frac { P+\rho (P) c^2}{r(r-2u)} \Big[ \frac {K}{2} P r^3 +u \Big] \tag {**}
\end {align*}
\end{document}

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