the graph of y = cos x and the line y = (-1)/2 over the interval [0,2 pi]. Where do the two graphs intersect? Give exact answers in radians,

the graph of y = cos x and the line y = (-1)/2 over the interval [0,2 pi]. Where do the two graphs intersect? Give exact answers in radians,

$$\small{\text{$ \begin{array}{rcll} \cos{(x)} &=& -\frac12 \\\\ \cos{(x)} &=& -\frac12 & \qquad | \qquad \pm\arccos{} \\\\ x_{1,2} &=& \pm \arccos{(-\frac12 )}\\\\ x_{1,2} &=& \pm 120 \ensurement{^{\circ}} \\\\ x_{1} &=& 120 \ensurement{^{\circ}} \\\\ x_1 &=& \frac{2}{3}\cdot \pi \\\\\\ x_2 &=& -120\ensurement{^{\circ}} + 360 \ensurement{^{\circ}} \\\\ x_2 &=& 240 \ensurement{^{\circ}} \\\\ x_2 &=& \frac{4}{3}\cdot \pi \end{array} $}}$$