I.10.32 Flux of the vector field

condition: Find the flow of the vector field \( \vec{a}=3 x \vec{\imath}-z \vec{\jmath} \) through the part of the surface \( S: x^{2}+y^{2}=4 \) cut out by the planes \( P_{1}: z=0, P_{2}: z=2 \), \( P_{3}: y=0,(y \geq 0) \quad \) using the Gauss-Ostrogradsky formula (normal external to a closed surface).