Answer:

Step-by-step explanation:
The given trigonometric equation is
.
We can either use the Pythagorean identity or the right angle triangle to solve for
.
According to the Pythagorean identity,

Recall that, the cosine function is an even function, therefore

.
We substitute this value in to the above Pythagorean identity to get;






But we were given that,
, so we choose the negative value.

The correct answer is B