You first need to solve the characteristic equation. This is the polynomial equation (for m, according to the book I was taught from, but you may naturally use whatever variable you like) given by the coefficients of the differential equation. Powers of m are given by the order of the term.
Solving this equation gives:
Now, each possible solution for m is going to give a linearly independent solution for the homogeneous equation of the form: where the is the "nth" possible value of m given by the characteristic equation. So in this case the most solution for the homogeneous differential equation will be:
There are other ways to write this, using sine and cosine to replace the complex exponentials, but I'll leave it this way as it exposes the structure behind the method.
(Note: the other two problems solve much more nicely than this one.)