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Here's the key:

| top row: a , b , c  middle row: d , e , f  bottom row: g , h , i | ... copy the first two columns on the outside to the right ... outside top row: a , b  outside middle row: d , e  outside bottom row: g , h
Copy the first two columns on the
outside... and be neat about it!

Now, you'll multiply down and add...

               | top row: a , b, c  middle row: d , e , f  bottom row: g , h , i | ... outside top row: a , b  outside middle row: d , e  outside bottom row: g , h ... multiply the a , e , and i ... + ... multiply the b , f , and g ... + ... multiply the c , d , h ... +


         = a e i + b f g + c d h ...


Then, multiply up and subtract...

                                                                          | top row: a , b, c  middle row: d , e , f  bottom row: g , h , i | ... outside top row: a , b  outside middle row: d , e  outside bottom row: g , h... multiply the g , e , and c ...- ... multiply the h , f , and a ... - ... multiply the i , d , and b ... -


= a e i + b f g + c d h - g e c - h f a - i d b

No, you don't have to memorize all these letters...  Just learn the process!