Answer
$$\frac{2\sqrt{3}}{3}$$
Work Step by Step
To evaluate trigonometric functions without a calculator, it is necessary to use the unit circle. Doing this, we find that the function evaluates to:
$$\frac{1}{\cos \left(-\frac{\pi }{6}\right)}\\ \frac{1}{\frac{\sqrt{3}}{2}}\\ \frac{2\sqrt{3}}{3}$$