I know there are some threat about multuple alignments. However, I couldnt find a solution to my simple problem. Here is a short reproducible example:
\documentclass[a5paper,oneside]{book}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}
\usepackage{mathtools}
\begin{document}
\begin{flalign*}
MSPE &= E[(y_{0}-\hat{f}(x_0))^2|X=x_0] \\
&= E[y_{0}^2] +E[\hat{f}(x_0)^2]-E[2y_{0}\hat{f}(x_0)] \\
&= Var[y_{0}] + E[y_{0}]^2 + Var[\hat{f}(x_0)] + E[\hat{f}(x_0)]^2 -E[2y_{0}\hat{f}(x_0)] \\
& \textnormal{Text should be left aligned unter the word MSPE}\\
&= Var[y_{0}] + Var[\hat{f}(x_0)] + (f(x)^2 - E[2y_{0}\hat{f}(x_0)]+ E[\hat{f}(x_0)]^2)
\end{flalign*}
\end{document}
The line with the text should be aligned under the word "MSPE" and all other equations that follows under
\textnormal{Text should be left aligned unter the word MSPE}
should be set normaly with &=