Using the chain rule leads to
If you want to find the derivative of it is by definition, so it is equal to one
-18 has no x in it, so it is a constant -> the derivative of a constant is 0
And in your example you do not need to use the product rule, because -18 is a factor of .
So the solution is
You can use the product rule -> but you need to use that [-18]' = 0.
Any more questions?