Now, since is given, you need to be able to express in this formula for in terms of .
And how do you do that? Tipp: sketch a right triangle with acute angle , adjacent leg and opposite leg . By Pyhtagoras's theorem you have that the length of the hypotenuse of this triangle is .
Therefore, by definition, we have . This is valid only for acute angles . In the general case, you need to write this as . The sign depends on the quadrant in which the point that corresponds to lies on the unit circle.
To sum it up you get:
If you are allowed to assume that is an acute angle, then you only take the case of . Now plug in the given value of ...