The horizontal line will create a segment; go here:
Circle Sector and Segment
Incidentally, there is a similar question in the Trigonometry forum, but it does not offer a precise solution (yet).What is the most simple way of going from this equation (i.e. what is the easiest way of solving it, obtaining an accurate, numerical value)?
It is not clear to me what the phrase "find... the line" means in the original question. It is possible that the distance of the bisecting line from the diameter is a nice number expression built from natural numbers, , the four arithmetic operations, radicals, and trigonometric functions. However, there may not be such an expression since the number of reals is vastly greater than the number of expressions (even though both are infinite). So, sometimes the description "the number such that " is as good as one can get. I would ask the instructor to clarify what kind of answer is expected. I believe that in this case it is not something like asking for a hint.
It is up to you now to decide what to do. What numerical approaches have you been taught? Do you know how to use technology (such as a graphics calculator) to solve an equation? These issues (that were not flagged in the original post) do not belong in the Geometry subforum.