You are using a instead of u. Minor difference but instead you need to find
2. Aside from my above note about a then yes.
3. You need to use the chain rule. It's a tricky one to spot but if you imagine so the derivative is multiplied by -1. Thus
For is true because you can write it as . x^2 is simple enough but you must multiply by -1/2 which gives -x
4. Almost, because of the sign error in finding your answer is a factor of -1 out.