Question: What is the area bounded by one petal of the graph r = (1 - Cos(3theta))?

From plotting this in the polar and Cartesian plane I gathered that one petal occurs on the interval (0, 2pi/3).

Therefore the area would be 1/2 * (Integral from 0 to 2pi/3 of (1-Cos(3theta))^2 which gives me approximately 4.72. However according to the online quiz this is not the correct answer. What am I doing wrong here? I even double checked with an online integration calculator and got the same value.