Write the Word or Phrase That Best Completes Each Statement $\theta$

Write the word or phrase that best completes each statement or answers the question.
Solve the problem.
-A water wheel rotates through the angle $\theta$ , the water level L behind the wheel changes according to the equation
$$\mathrm { L } = 1 - \sin \theta - 2 \cos ^ { 2 } \theta$$
where $\mathrm { L }$ is measured in inches. Determine the values of $\theta$ for which the water level is zero. Find the exact values. Do not use a calculator.