At first you should find the general solution of the homogenous equation.Then you should find the particular solutions of the in-homogenous equations. I use plural words because you have in fact 25 in-homogenous equations with the driving functions being the terms in the Fourier series. That's because the equation is linear and so you can just consider each term the only one which is there and find the particular solution corresponding only to that term and then add the particular solutions together and to the general solution of the homogenous equation to get the answer.
it will be very laborious right by hand calculation? is it possible to solve on matlab by writing code?
You're not going to actually solve 25 differential equations!
Just solve it with n,without giving it specific values,Which means you're going to solve only 2 differential equations one of which is the representative of 24 differential equations.
But yes,you can solve it with softwares like MatLab too.