Answer:
Jamie's mistake: The last term
and not
. Jamie multiply the signs incorrectly.
Advice to avoid this mistake in the future: Jamie should remember that when two negative numbers are multiplied, the result is a positive number.
Explanation:
The correct procedure is the one shown below:

You can identify that the last term is actually
and not
, so Jamie multiply the signs incorrectly.
To avoid that mistake in the future, Jamie must remember the mulitplication of signs:

So when two negative numbers are multiplied, the result is a positive number.