Given number = 97215x6

(6 + 5 + 2 + 9) - (x + 1 + 7) = (14 - x), which must be divisible by 11.

Hence  x = 3

(11² + 12² + 13² + ... + 20²) = (1² + 2² + 3² + ... + 20²) - (1² + 2² + 3² + ... + 10²)

	Ref: (12 + 22 + 32 + ... + n2) =	1	n(n + 1)(2n + 1)
		6

=		20 x 21 x 41	-	10 x 11 x 21
		6		6

= (2870 - 385)

= 2485.

6 = 3 x 2. Clearly, 5 * 2 is divisible by 2. Replace * by x.

Then, (5 + x + 2) must be divisible by 3. So, x = 2.

(xⁿ - 1) will be divisibly by (x + 1) only when n is even.

(49¹⁵ - 1) = {(7²)¹⁵ - 1} = (7³⁰ - 1), which is divisible by (7 +1), i.e., 8.