Use reduction of order. Given a solution
, look for a second solution of the form
.
Compute the first two derivatives of
:

Substitute them into the ODE:

Now substitute
and you end up with a linear ODE:

Multiply through both sides by
(if you're familiar with the integrating factor method, this is it):

Bear in mind that in order to do this, we require
. Just to avoid having to deal with absolute values later, let's further assume
.
Notice that the left side is the derivative of a product,

Integrate both sides with respect to
:

Solve for
:

Solve for
by integrating both sides:

Integrate by parts:

Solve for
:

But since
is already accounted for, the second solution is just

Still, the general solution would be
