Hi Sonia! Here is how we obtain dy/dx for that expression. I have included two different approaches to obtaining dy/dx. Next up is the remaining part the stationary points and their nature.

At the turning points, remember: the tangent to the curve at that point will always be horizontal, with a gradient of 0, so dy/dx = 0 at these points.

You have converted a differentiation question into a trigo question which you will need the trigo knowledge which you have learnt.