[SOLVED] Unsure - Surface Integral Q?

Hey everyone!

I have the following question:

In the following cases you are given a 3D velocity vector field. This represents the velocity of a fluid at the position (x,y,z). Find the volume of the fluid that is flowing out of the unit cube per second. The density of the fluid is 1g/m^3. Use the simplest possible method to solve when __v__ = __i__ + __j__ - __k__.

I am unsure about what method to take. We are currently learning about vector fields, Green's Theorem, etc. So I'm assuming the method is somehow similar to these. Would it be a surface integral and if so how would I go about solving it?