Thread: proving big-o

I am doing some math for a comp sci course.

I was just wondering if anyone could explain how exactly they came about deriving C from the following examples on page 2 and 3 of the link.

http://www.cs.utsa.edu/~bylander/cs3233/big-oh.pdf

Thank you so much in advance!!!

What exactly do you not understand about these examples? They show how C is found. For example, the first examples says , so one can take C = 4 (and also 5, 6, etc.).