continuing equation under the equal sign

Question

how to continued the equation under the equal sign?

I want to do it like this

Answer 1

\documentclass[preview,border=12pt]{standalone}
\usepackage{mathtools}
\begin{document}
\abovedisplayskip=0pt\relax
\begin{align*}
a + b + c &= d + e + f\\
a + b + c
&=
\!
\begin{aligned}[t]
& d + e + f\\
& + g + h\\
\end{aligned}\\
a + b + c
&=
\!
\begin{aligned}[t]
& d + e + f\\
& \phantom{d} + g + h\\
\end{aligned}
\end{align*}
\end{document}

Answer 2

You can use align environment:

\documentclass[varwidth]{standalone}

\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}

\begin{document}
    \begin{align*}
        f(x_1, x_2, x_3) = & f_0 m_0 \vee f_1 m_1 \vee f_3 m_3 \vee \\
          & \vee f_4 m_4 \vee f_5 m_5 \vee f_6 m_6 \vee f_7 m_7 \\
        = & 0 m_0 \vee 1 m_1 \vee 0 m_3 \vee \\
          & \vee 1 m_4 \vee 0 m_5 \vee 0 m_6 \vee 1 m_7
    \end{align*}
\end{document}