Mathematics font: What font is this?

For my humble opinion it is

\usepackage{newtxtext}
\usepackage[lite]{mtpro2}

Here a screenshot taken from the documentation of newtx pag. 17:

And here's a reproduction of the first long equation in the screenshot you posted:

This is the link to install the font mtpro2 v. lite (hence it is not complete).

\documentclass{article}
\usepackage{newtxtext} % Times Roman clone text font (not crucial)
\usepackage[zswash,lite]{mtpro2} % math font package
\begin{document}
\[
\int_{-\infty}^{\infty} \frac{e^{-3ix}}{2(x^2+4)}dx+0
   =-2\pi i \lim_{z\to-2i}(z+2i)f_2(z)
\]
\end{document}

Here's your font according to @ivankokan \usepackage{newtxtext}and \usepackege{newtxmath}

\documentclass{article} 
    \usepackage{newtxtext}
    \usepackage{newtxmath}  
   \begin{document}
However, the combination \[\int_{-\rho}^{\rho} f_2(x)dx+\int_{C_\rho^-}f_2(z)dz\] equals \textit{minus} $2\pi \mathrm{Res}(f_2 ;-2i)$, because$\cdots$ 
   \end{document}

And here's your text! good luck!