Outside: The first term of the first binomial and the last term of the second binomial:
Inside: The last term of the first binomial and the first term of the second binomial:
Last: The last terms of each binomial:
Add 'em up:
Combine like terms:
Done.
Now there is nothing really sacred about the order of the multiplications. You could just as easily, and correctly, done Outside, Inside, First, Last or Last, First, Outside, Inside. It is just that OIFL and LFOI don't spell anything that a student is likely to remember.
