ok....so do you know how to turn into an exact answer?
Assuming you do, the answer is
so plugging that into the equation, you get
simplifying, you get
that then get simplified further to
and finally
the initial equation is , which puts it in 3rd quadrant, so we know it needs to be negative, or
Is it clearer?
that's the most simple exact answer. You could actually solve it and get a decimal value, but it wouldn't be exact any more since you'd have to round it. The (inaccurate) decimal answer is like -0.9659258263.... but if this is for class and your professor/teacher is anything like mine, giving a decimal answer would result in a fail.
*edit* Good call Jhevon. There are certainly other ways to solve this equation.