11.5.3.1 With variable coefficients

Problem: Find the solution of the second order linear equation, satisfying the given initial conditions (the Cauchy problem). \[ \begin{array}{l} z_{x x}^{\prime \prime}-2 \sin x \cdot z_{x y}^{\prime \prime}-\cos ^{2} x \cdot z_{y y}^{\prime \prime}-z_{x}^{\prime}+(\sin x-\cos x-1) z_{y}^{\prime}=0, \\ \left.z\right|_{x=0}=3 y,\left.\quad z_{x}^{\prime}\right|_{x=0}=5 \end{array} \]