Here's the rest of the solution if it helps explain the previous sentence:

...But A1 is 1 less than 10^666; and so if B1 is multiplied by A1, the result is the same as multiplying B1 by 10^666 (which yields an integer composed of 666 digits 2 followed by 666 zeros) and subtracting the integer B1. It clearly follows that the result will be a number of form:

{22.....21} {77.....78}

__665________665__

I really dont understand what the original line means:

"Since 6 = 3 x 2, the product in question will be the same as that obtained by multiplying the integer A1 composed of 666 digits 9 by the integer B1 composed of 666 digits 2"