So the questions asks find all the values of x in interval [0,2pi)

for the following equation

So the way I go about solving it is I subtract 12cos(x) to the other side and replace cos(2x) for the double angle identity which gives me:

So essentially, I distribute the 4 and rewrite the equation as follows:

Next I take out the common factor of 4 and factor out the whole equation:

to

After solving each one I get the following answers:

However, the online assignment guide is telling me I've gotten the wrong answers. I'm honestly lost, can anyone tell me where exactly I'm getting this wrong?