Chapter 7 - Trigonometry and Periodic Functions - 7.8 Inverse Trigonometric Functions - Exercises and Problems for Section 7.8 - Exercises and Problems - Page 313: 10

No solution.

First, subtract $6$ from both sides to get $$2\cos \theta = 3$$ Divide both sides by $2$ to get $$\cos \theta = 3/2$$ However, $\cos \theta$ is bounded between $-1$ and $1$. $3/2$ is greater than $1$ ($2/2$) so there is no solution.