Can you write out what the sum of b and d are? Try it. Use pencil and paper here if you want.
Next, if sum(b) or sum(d) were zero, then you could not multiply by those quantities. But if that were true, then you could not have a finite ratio as a result. So it is safe to multiply by those quantities. You should probably verify the result at the end to ensure a singularity does not result anyway.
Having done that wee bit of algebra, can you write down the equations in a simple, standard form? Can you solve them by hand? If not, then there are tools that can do this in MATLAB. But just a bit more simple algebra on paper will suffice.
I would point out that it is never a good idea to rely so much on a computer program that you forget how to do simple algebra yourself (or never learn basic algebra at all.) So try it! If this is really a homework problem, then try using the symbolic toolbox. Or learn how backslash would be used to solve the problem in the end. But if it is homework, then I we should not be doing it for you anyway.
