TeX source:

\begin{align*} \mathbf{x}_1\cdot \mathbf{y} &= m_1 \mathbf{x}_1\cdot\mathbf{x}_1 + \dots + m_p \mathbf{x}_1\cdot\mathbf{x}_p + c \mathbf{x}_1\cdot\mathbf{u} \\ & \vdots \\ \mathbf{x}_p\cdot \mathbf{y} &= m_1 \mathbf{x}_p\cdot\mathbf{x}_1 + \dots + m_p \mathbf{x}_p\cdot\mathbf{x}_p + c \mathbf{x}_p\cdot\mathbf{u} \\ \mathbf{u}\cdot \mathbf{y} &= m_1 \mathbf{u}\cdot\mathbf{x}_1 + \dots + m_p \mathbf{u}\cdot\mathbf{x}_p + c \mathbf{u}\cdot\mathbf{u} \end{align*}