If I correctly recall my E&M studies from 1958,
I suspect that this is a chump question.
The E flux through ANY closed surface is equal to the net
charge inside, or maybe it's the net charge divided by ε₀ .
Yep, I think that's it.
Your charges are both positive, so if they're inside your cube,
the net charge inside will be 17.1μC .
Your cube has all faces 5 units from the axes, and neither charge
has an x, y, or z component with absolute value greater or equal
to 5. So both charges are well inside the cube.
Seems to me, then, like the total E flux through the cubical
17.1μC / ε₀ .
I don't really care whether this is helpful or not.
I just hope it's correct.