If you require further help, you should post any understanding at all you have of the problem. That way, we can tailor it more to what you actually require. For now, here are some hints.

a) Differentiate it twice and then pull out the relevant constant as a factor. This is a very common question in the exam and is free marks in the long run.

b) Rely on the equation in part a) to do this part. There may be some simultaneous equations to solve. (Alternatively, you could use \(v^2=n^2(a^2-x^2) \) if you prefer, but I haven't checked that it will work using this approach.)

c) For trigonometric functions, the maximum value of \(\sin x\), \(\cos x\), \(-\sin x\) and \(-\cos x\) are 1, which you can assume and use here.