I'd like to cross out a subsection of an aligned environment, as below (the actual content doesn't matter).
\documentclass{article}
\usepackage{amsmath,cancel}
\begin{document}
\begin{align}
4h + b &= \lambda 2bh \
4 \cancel{b} &= \lambda b^{\cancel{2}} && \quad \text{for } b \neq 0\
b ^{2}h - V &= 0\
\
% cross out this section
b &= \frac{4}{\lambda}\
4h + \frac{4}{\lambda} &= 2\cancel{\lambda} h \frac{4}{\cancel{\lambda} }\
4 \lambda h+ 4 &= 8 \lambda h\
4 \lambda h &= 4\
\lambda h &= 1
% end the crossed out section
\end{align}
\end{document}
I tried relative stacking on a phantom copy of the math (as in this answer), but it seems like \llap only stacks up one line. I really have no idea what I'm doing, but I hope there's a solution that doesn't involve something too heavy like tikz. Something like the \Aboxed solution to the same issue with boxed would be incredible.
The thing I tried:
\begin{align*}
4h + b &= \lambda 2bh \\
4 \cancel{b} &= \lambda b^{\cancel{2}} && \quad \text{for } b \neq 0\\
b ^{2}h - V &= 0\\\\
% cross out this section
b &= \frac{4}{\lambda}\
4h + \frac{4}{\lambda} &= 2\cancel{\lambda} h \frac{4}{\cancel{\lambda} }\
4 \lambda h+ 4 &= 8 \lambda h\
4 \lambda h &= 4\
\lambda h &= 1
% end the crossed out section
% the code to generate a phantom cross out
; \llap{\mathrel{\xcancel{\phantom{\begin{align}
b = \frac{4}{\lambda}\
4h + \frac{4}{\lambda} = 2\cancel{\lambda} h \frac{4}{\cancel{\lambda} }\
4 \lambda h+ 4 = 8 \lambda h\
4 \lambda h = 4\
\lambda h = 1\
\end{align}}}}}
\end{align*}
