Easy math, but I'm dumb

The drain/filling rates are not dependant on the volume. They are constants.

Which is exactly why it is not a Calculus problem.

I don't know if this counts as Linear Algebra, but you sure can put the problem under the "vector view".
If you're going EAST 10 miles per hour, but something is pulling you back at 5 miles per hour, you'll end up with 5 miles per hour (or less due to fraction).
The same goes for Volume-filling, the filling rate of the pump ADDs water to the container, thus it is positive in algebraic expression (+ V1), the draining TAKES AWAY water from the container, thus it is negative (-V2). Find the combination between those two (V1 + -V2) and you can figure out where the thing is headed, and how fast.