\usepackage[a4paper, total={6in, 8in}]{geometry}
$$ KP = e^{-\lambda} \Big( \frac {\nu'}{r} +\frac {1}{r}\Big) -\frac {1}{r^2}$$
    \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 ?

  • 1
    Please read tex.stackexchange.com/questions/503/why-is-preferable-to on the $$...$$ topic – user31729 Aug 7 '14 at 6:29
  • 1
    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}. – user31729 Aug 7 '14 at 6:29
  • 2
    Put both of them in an align environment and use ampersands & to mark positions. – Johannes_B Aug 7 '14 at 6:40
  • 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 7 '14 at 7:10
  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*}

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