# writing text beside an equation

I want to write text beside an equation to appear like the attatched photo. However, i'm keeping have an error message:

! Misplaced alignment tab character &. l.153 cj

(\gamma_T;X,T)\delequal &

                                \text{the number of firing times of reac.


can any one help ?? here is my code:

\begin{document}
\usepackage{amsmath}
\begin{align} cj (\gamma_T;X,T)\delequal & \text{the number of firing times of reaction R_j} \text{ in the time interval [T,T_\gamma ] , given X(x)=T }.\\ \end{align}
\end{document}

• As is, your code is not compilable. – Bernard Sep 22 '16 at 0:03

Your text is different from suggested one and not compilable. In particular, please observe that your big T is small \tau and T_\gamma is \delta\tau. Here is the code of the text from your picture, up to the equation number and your formulation of text part:

\documentclass{article}
\usepackage{amsmath}

\def\delequal{\stackrel{\triangle}{=}} % ?

\begin{document}
%\begin{align} \begin{gathered} n_j (\delta\tau;\mathbf{x},\tau)\delequal \text{the number of firing times of reaction}\\ R_j\text{ in the time interval [\tau,\tau+\delta\tau ], given \mathbf{X}(\tau)=\mathbf{x}}. \end{gathered} %\end{align}
\end{document}


Like this?

\documentclass{article}
\usepackage{amsmath,amssymb}

\begin{document}
\begin{aligned} cj (\gamma_T;X,T)\triangleq & \text{ the number of firing times of reaction R_j} \\ & \text{ in the time interval [T,T_\gamma ] , given X(x)=T }. \end{aligned}
\end{document}