Gauss law:

$\overline{){{\mathbf{\varphi}}}_{{\mathbf{E}}}{\mathbf{=}}\frac{{\mathbf{q}}_{\mathbf{e}\mathbf{n}\mathbf{c}}}{{\mathbf{\epsilon}}_{\mathbf{0}}}}$

The three small spheres shown in the figure (Figure 1) carry charges q1 = 4.35nC , q2 = -8.00nC , and q3= 2.25nC.

Find the net electric flux through the closed surface S1 shown in cross-section in the figure.