How to center align part of an equation within alignat?

Question

I'm using the environment alignat to align my set of of equations at two different points. Now, I dot not only want to align it these two points, but also want to have everything in between these align-point centered.

currently it looks like:

x = y                    + z
a = b + blablablablablab + u

But I want to look like:

x =           y          + z
a = b + blablablablablab + u

my current code looks like:

\centering
\begin{alignat*}{2}
x &= y &+ z\\
x &= b+blablablablablab &+ u
\end{alignat*}

Thanks for your help!

Answer 1

A possible solution with \mathmakebox from mathtools:

\documentclass[a4paper]{article}
\usepackage{mathtools}

\newlength{\mymathln}
\newcommand{\aligninside}[2]{
  \settowidth{\mymathln}{#2}
  \mathmakebox[\mymathln]{#1}
}

\begin{document}

\begin{alignat*}{2}
x &= \aligninside{y}{b+blablablablablab} & &+ z\\
x &= b+blablablablablab & &+ u
\end{alignat*}

\end{document}

The result: