Okay, you can bring the 0.5 to the right hand side and you're left with cos(3x) = -0.5. You know, or should be familiar with the fact that the arc-cosine of 0.5 is 60 degrees, or pi/3 radians. So, because the value is negative, it's in the second quadrant and you can say that it's 120 degrees, or 2*pi/3 radians. Now, you're left with 3x = 2*pi/3. Divide by 3, and see the values that come out, and you should get about 0.69813... radians.