Use the divergence theorem,
[tex]\displaystyle\iint_{\partial\sigma}\mathbf F(x,y,z)\cdot\mathrm d\mathbf S=\iiint_\sigma\mathrm{div}\mathbf F(x,y,z)\,\mathrm dV[/tex]
We have
[tex]\mathrm{div}\mathbf F(x,y,z)=\dfrac{\partial(3x)}{\partial x}+\dfrac{\partial(2y)}{\partial y}+\dfrac{\partial0}{\partial z}=5[/tex]
so that the flux across [tex]\sigma[/tex] is equal to 5 times the volume of the cube. The cube itself has edge length 5, so its volume is [tex]5^3=125[/tex], making the flux [tex]5^4=\boxed{625}[/tex].
