Clearly,
Then the number is divisible by 21.
What is the value of 9 - 2? Note that "9 - 2" is multiplied on some other factor, the exact value (and even expression) of which we are unclear. Since this value is unknown, we can compress the (very long) expression with "a".
Note that the other term is "7 * 2^y", where "y" is unknown, and thus "2^y" is unknown. This expression may be replaced with "b".
Then the sum is 7a + 7b = 7(a + b). Replace "a + b" with "c".