11.6.3 Systems of equations in partial derivatives of the first order

Problem: Determine the type of the system of linear partial differential equations of the first order: \[ \left\{\begin{array}{l} 3 u_{x}+4 u_{y}-v_{y}+v_{x}=0 \\ u_{x}+v_{y}-v_{x}=0 \end{array}\right. \]