I have a Question in regards to a Power series questions that I have almost completed.

My Question looks like this:

The function f(x)= 5xarctan(4x) Find:

C1: Which I found to be 0

C2: Which I found to be 0

C3: Which I found to be 20

C4: WHICH I CANT FIGURE OUT, I thought it was 20x^4/3 But it is not right.

As well Find R: Which I cant figure out...

If anyone could help that would be great!