This question is possibly the very first in your course about fluid mechanics, a direct application of what you've learnt.
The only advice I'm willing to give is: do not stick to 3D calculus, which is rarely useful and makes everything complicated or impossible. Here the situation is 1D. Write the forces on a volume element, deduce the rest.
Also: the flow is probably laminar in this problem (we don't see any drawing here). It is sometimes the case in real life, but not so often. So, do not generalize.