Use the Parallelogram Rule to Find the Magnitude of the Resultant

Use the parallelogram rule to find the magnitude of the resultant force for the two forces shown in the figure. Round to
one decimal place.
-Explain what is wrong with the following solution for the equation $\cos 2 \theta = \sqrt { 3 }$ in the interval $[ 0,2 \pi )$ .
$$\cos 2 \theta = \sqrt { 3 }$$
$$\cos \theta = \frac { \sqrt { 3 } } { 2 }$$
$$\theta = \frac { \pi } { 6 } \text { or } \theta = \frac { 11 \pi } { 6 }$$