Well, there are a couple of things I think when I look at it.

A first step might be to try to understand the expressions defining M+s and N-r. Can you expand the brackets? That won't necessarily help you to do the question, but if you've not tried anything it's something you know to try if you're stuck.

If that's no use, think about *why* the restriction on the values of b matters. In general, it will certainly be possible for M to be nonzero, so it must be something about these special values of b that forces M to be 0. Can you take the inequality b < 2a-1 and use it to come up with an inequality relevant to the value of M?

In general, when you're stuck on where to start with a question, it's a good idea to collect all of the information that you know, and see what you can apply to that information (be it expanding brackets, applying theorems, sketching curves) even if you're not sure where you'll end up - you might learn something.