\nonstopmode
\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{cancel}
\pagestyle{empty}
\begin{document}
$$
p_{{\mathrm  {total}}}\equiv {\frac  {1}{A_{{\mathrm  {F}}}}}{\biggl [}P_{{\mathrm  {gas}}}+P_{{\mathrm  {deg}}}+P_{{\mathrm  {rad}}}{\biggr ]}.
$$
\end{document}
