\documentclass{article}
\usepackage{amsmath}
\usepackage{newtxtext,newtxmath}
\begin{document}
\begin{align*}
h(x,y) &=
\iint\limits_{(\theta,f)\in[0,\pi]\times[0,\infty)}
F(f,\theta) e^{j2\pi f(x\cos\theta+y\sin\theta)}f\,df\,d\theta
\\
&+
\iint\limits_{(\theta,f)\in[0,\pi]\times(-\infty,0]}
F(f,\theta) e^{j2\pi f(x\cos\theta+y\sin\theta)}(-f)\, df\, d\theta
\end{align*}
Using $\lvert f\rvert$ we can combine the above two terms into one
\begin{equation}
h(x,y)=
\int_{0}^{\pi}
\left[\int_{-\infty}^{\infty}