multistep problem involving f''(x) and horizontal tanget lines

Exactly as is

1. Given the function f(x) = $\displaystyle sqrt{1-x^2}$ find the points on the domain of f at which f'(x) is zero. Also find the points on the domain at which f''(x) is zero, when possible give exact answers.

2. repeat for f(x) =$\displaystyle 4sqrt{3}sin(x) + 2cos(2x)$ where $\displaystyle 0 <= x < 2pi$. again when possible exact answers should be given.

3. Find the equation of the tanget line to the graph $\displaystyle Sin(xy^2) + x^2 = (x + y)^4$

In regards to number 1, I should derive it and find where y = 0; and then to find f''(x) I would find the derivative squared?

Assuming that logic is correct, I would repeat for number two with the new info.

3, I am not focusing on yet but, it should just be finding the derivative and then creating an equation...