When do we use the sum rule?
Are there multiple terms in the function? If so, apply the sum rule. Differences work as well since they can easily be converted to sums. Also repeated additions can be combined into one step where all the derivatives of each term are added together.
How do we take derivatives of sums?
The sum rule is so easy that most of the time you will not even realize that you are doing anything. Once you have identified the terms being added or subtracted then take the derivative of each term. Taking these derivatives will be the hard part. Then once you have all of the derivatives just add them or subtract them back in the same order as in the original function.