these are all local minima or maxima so set the derivative equal to zero an solve for x. You'll get a few solutions and you should be able to eyeball the correct ones.
The picture shows a portion of the graph of the periodic function
f(x) = sin(1 + sinx). Find the x-coordinates of the turning points P, Q, and R. Round the answers to three decimal places. Hint: The x-coordinate of Q is halfway between the x-coordinates of P and R.