Variable-Size Matrix in Matlab

Hey guys! I've been on Mathworks dozens of times to seek help with my Matlab issues, but this is my first actual post. Cheers to firsts! Anywho, here is my problem: I am given an initial equation that is shown in the picture attached:

As can be read in the problem statement, I am to create a program to essentially gather values for the flux, which is represented by phi. I am given the boundary conditions of -D(delta_phi/delta_r) = 0 at r = 0, and phi = 0 at r = R (which is the furthest boundary and is given by a user input). I know that I need to set up a loop to create a matrix of N-1 size, but I am having trouble with incorporating the equation into this matrix. I know that I must use a linear system of equations, but I'm lost on how to start. The values for D, k, sigma_a, and vSigma_f are given numbers, and n and N are given by inputs. I'm not asking for anyone to figure this out for me. I just really need someone to point me in the right direction or give me a running start, because my professor has a tendency of not answering his email for days. Thanks in advance to anyone who answers!