I think we are just supposed to simplify the expression on the right side of the equation. The instructions are to "express y in terms of x"
Then you're not supposed to simplify; you're supposed to solve for y.
A good start will be to apply the Pythagorean Identity to the quadric terms. For instance, restate (cos2
in terms of sine, and then combine with the other sine term inside that square root. And so forth.
Once you have the right-hand side simplified somewhat, you can take the inverse.