Alignment within align environment

Question

My minimum working example:

\documentclass{report}

\usepackage{amsmath}

\begin{document}

\begin{align}
x_1 = \alpha,\, x_2 = q,\, u = \delta \\
f_1(x_1) &= C_1 \left [ \phi_{z1}(x_1) + \phi_{z2}(x_1)M \right ] \\
f_2(x_1) &= C_2 \left [ \phi_{m1}(x_1) + \phi_{m2}(x_1)M \right ] \\
g_1 = C_1 b_z,\, g_2 = C_2 b_m \\
C_1 = \frac{\bar{q}S}{mV_T},\, C_2 = \frac{\bar{q}Sd}{I_{yy}}
\end{align}

\end{document}

The result:

What I want:

Answer 1

As far as I understand, you want to obtain C_1 in the last line moved to the left. (If no, simply remove \!). However, I would suggest replacing ,\,'s by ,\quad's.

\documentclass{report}

\usepackage{amsmath}

\begin{document}

\begin{align}
&x_1 = \alpha,\, x_2 = q,\, u = \delta \\
&f_1(x_1) = C_1 \left [ \phi_{z1}(x_1) + \phi_{z2}(x_1)M \right ] \\
&f_2(x_1) = C_2 \left [ \phi_{m1}(x_1) + \phi_{m2}(x_1)M \right ] \\
&g_1= C_1 b_z,\, g_2 = C_2 b_m \\
&\!C_1 = \frac{\bar{q}S}{mV_T},\, C_2 = \frac{\bar{q}Sd}{I_{yy}}
\end{align}

\end{document}

Answer 2

Przemyslaw's answer gives the output you want, but in the case that your set of equations becomes more complex (e.g. you want to align two sets of two equations with different alignment points), you could use aligned inside gather for just the two you want aligned:

\documentclass{report}

\usepackage{amsmath}

\begin{document}

\begin{gather}
x_1 = \alpha,\, x_2 = q,\, u = \delta \\
\begin{aligned}
f_1(x_1) &= C_1 \left [ \phi_{z1}(x_1) + \phi_{z2}(x_1)M \right ] \\
f_2(x_1) &= C_2 \left [ \phi_{m1}(x_1) + \phi_{m2}(x_1)M \right ] \\
\end{aligned}\\
g_1 = C_1 b_z,\, g_2 = C_2 b_m \\
C_1 = \frac{\bar{q}S}{mV_T},\, C_2 = \frac{\bar{q}Sd}{I_{yy}}
\end{gather}

\end{document}