My MWE:
\documentclass{report}
\usepackage{amsmath}
\begin{document}
The partial derivatives are given by:
\begin{subequations}
\begin{align}
\frac{\partial \beta_{2,1}}{\partial x_1} &= \gamma_2x_2 \cos(x_1), &&\frac{\partial \beta_{2,1}}{\partial x_2} = \gamma_2 \sin(x_1), &&&\frac{\partial \beta_{2,1}}{\partial u} = 0\\
\frac{\partial \beta_{2,2}}{\partial x_1} &= 0, &&\frac{\partial \beta_{2,2}}{\partial x_2} = \gamma_2 x_2, &&&\frac{\partial \beta_{2,2}}{\partial u} = 0\\
\frac{\partial \beta_{2,3}}{\partial x_1} &= 0, &&\frac{\partial \beta_{2,3}}{\partial x_2} = \gamma_2 u, &&&\frac{\partial \beta_{2,3}}{\partial u} = \gamma_2x_2
\end{align}
\end{subequations}
\end{document}
The result:
However, I would like to obtain the following result: