Quote by Genericcoder
I am sometimes confused by electric potential sometimes in question. I know its is the work done or potential energy of a particle at specific place,and electric potential is work done per charge.

You've said electric potential twice. So I think you meant to say electric potential energy in the first sentence. In this case, yes, you've understood it.
Quote by Genericcoder
For problem 2 I just used that U = Ufinal  UInititial; and since UInitial is approaching zero,so I neglected that.

This is the correct way to do the problem. But your method to get Ufinal was not right. (Although you did get the right answer anyway).
Quote by Genericcoder
...I think I solved it thanks for that hint BruceW I first we know since its conservative force then potential will transform to kinetic...W = 4.48 * 10^15 * 4 = 1.792 * 10^14;

You've got the right idea, but the calculation has a mistake. The plates are separated by 4 cm, so you should convert that into SI units before you use it in the equation, or it'll get complicated to keep track of the units.
Quote by Genericcoder
For problem 3 we know that V = kQ / d and we derived that from integration I saw lecture of walter lewin on that. We also know that U / q = V,so we can do the following > V = Vfinal  Vinitial , but I am still confused into how to think about this problem though I think I am over complicating it in my head..

You're given the electric field, so you need to relate that to the potential energy. The equation V = kQ / d is true for an electric field created by a point charge, but it is not true in this case, where they are telling us that the electric field is constant throughout space. In other words, you have a much simpler integration to do!