Solve 4(cos^2(3x)) − 3 = 0 for [0, 2π). Give exact answers.

Here is what I've done so far:

4 (cos^2(3x)) - 3 = 0

4 (cos^2(3x)) = 3 (added 3)

cos^2(3x) = 3/4 (divided by 4)

The three is throwing me off a little and I don't know what the next step is.

Please let me know if I can give anymore information or make anything clear.