Not really easy, but doable, with the help of a variant of my macro \Cen
from https://tex.stackexchange.com/a/209732/4427
There's a small catch: the width of a subscript k is not the same as for n, so we need a trick.
I also improved the appearance and the input of those expressions with commands from mathtools
; don't abuse \left
and \right
: as you'll see, the sizes of the delimiters are better here.
\documentclass[a4paper,titlepage]{article}
\usepackage{amsmath,amssymb,mathtools,calc}
\newcommand*{\PartialConj}{\bar{\partial}}
\DeclarePairedDelimiter{\Abs}{\lvert}{\rvert}
\DeclarePairedDelimiter{\Paren}{(}{)}
\DeclarePairedDelimiter{\Brace}{\{}{\}}
\makeatletter
\newcommand{\Shove}[3][c]{%
\ifmeasuring@
#3%
\else
\makebox[\ifcase\expandafter #2\maxcolumn@widths\fi][#1]{$\displaystyle{#3}$}%
\fi
}
\makeatother
% a subscript k as wide as n
\newcommand{\kn}{{\mathmakebox[\widthof{$\scriptstyle n$}]{k}}}
\begin{document}
\begin{align*}
\Abs{\PartialConj F(z)}
& \leqslant \Abs[\bigg]{\chi'\Paren[\bigg]{\frac{\Abs{z-\alpha_{i}}^{2}}{\delta_{i}^{2}}}}
\cdot
\frac{\Abs{P_{i}(z)}}{\delta_{i}}
+ \Abs[\bigg]{\chi'\Paren[\bigg]{\frac{\Abs{z-\alpha_{\kn}}^{2}}{\delta_{k}^{2}}}}
\cdot
\frac{\Abs{P_{\kn}(z)}}{\delta_{k}} \\
&\Shove[r]{2}{
{}+ \Abs[\bigg]{\chi'\Paren[\bigg]{\frac{\Abs{z-\alpha_{n}}^{2}}{\delta_{n}^{2}}}}
\cdot
\frac{\Abs{P_{n}(z)}}{\delta_{n}}
}\\
& \lesssim \sup_{z\in A_{i}}\Brace[\bigg]{\frac{\Abs{P_{i}(z)}}{\delta_{i}}}
+ \sup_{z\in A_{k}}\Brace[\bigg]{\frac{\Abs{P_{k}(z)}}{\delta_{k}}}
+ \sup_{z\in A_{n}}\Brace[\bigg]{\frac{\Abs{P_{n}(z)}}{\delta_{n}}}
\end{align*}
\end{document}

Compare it with a more standard way
\documentclass[a4paper,titlepage]{article}
\usepackage{amsmath,amssymb,mathtools,calc}
\newcommand*{\PartialConj}{\bar{\partial}}
\DeclarePairedDelimiter{\Abs}{\lvert}{\rvert}
\DeclarePairedDelimiter{\Paren}{(}{)}
\DeclarePairedDelimiter{\Brace}{\{}{\}}
\makeatletter
\newcommand{\Shove}[3][c]{%
\ifmeasuring@
#3%
\else
\makebox[\ifcase\expandafter #2\maxcolumn@widths\fi][#1]{$\displaystyle{#3}$}%
\fi
}
\makeatother
\newcommand{\kn}{{\mathmakebox[\widthof{$\scriptstyle n$}]{k}}}
\begin{document}
\begin{align*}
\Abs{\PartialConj F(z)}
& \leqslant \Abs[\bigg]{\chi'\Paren[\bigg]{\frac{\Abs{z-\alpha_{i}}^{2}}{\delta_{i}^{2}}}}
\cdot
\frac{\Abs{P_{i}(z)}}{\delta_{i}}
+ \Abs[\bigg]{\chi'\Paren[\bigg]{\frac{\Abs{z-\alpha_{\kn}}^{2}}{\delta_{k}^{2}}}}
\cdot
\frac{\Abs{P_{\kn}(z)}}{\delta_{k}} \\
& \qquad{}+ \Abs[\bigg]{\chi'\Paren[\bigg]{\frac{\Abs{z-\alpha_{n}}^{2}}{\delta_{n}^{2}}}}
\cdot
\frac{\Abs{P_{n}(z)}}{\delta_{n}}\\[1ex]
& \lesssim \sup_{z\in A_{i}}\Brace[\bigg]{\frac{\Abs{P_{i}(z)}}{\delta_{i}}}
+ \sup_{z\in A_{k}}\Brace[\bigg]{\frac{\Abs{P_{k}(z)}}{\delta_{k}}}
+ \sup_{z\in A_{n}}\Brace[\bigg]{\frac{\Abs{P_{n}(z)}}{\delta_{n}}}
\end{align*}
\end{document}

\Abs
,\PartialConj
).\documentclass
, the required packages, and other missing definitions.