Trapezoidal rule errors

I'm trying to calculate the upper error bound for the trapezoidal rule of the function 1/(8+x) from -2 to 1. EDIT n=4
I come up with .28125

Can someone please explain in depth the step to find K? I understand that you're supposed to maximize the second derivative of the function, but then the example problem in my book in which the second derivative is (2/x^3) from 1 to 2 sets K = 2 which clearly isn't the maximum. Really all I need to know is exactly what you are plugging in to find K.

This is very frustrating as the formula is very simple. There is very little material on the web to get a grasp on this stuff and the book is terrible and explains nothing. Thanks for the help in advance!