there are also three links given in post #2, it may do you well to follow them. see if you get the idea of how to go about solving these.
Let (of course you would write these as power series).
B. Find a particular solution to the original differential equation by using the MacLaurin series for f(x) = cosh(x), and give the general solution y = yh + yp
NOTE: Use the recurrence relation to find the first 4 non-zero terms of the series.
find and and plug them into the original diff eq. on the right, write the power series for . then, continue much as you did in solving the homogeneous case, but this time, you will equate the coefficients of like powers on both sides, as opposed to making the coefficients zero.
good luck, come back if you have problems