As per the divisibility rule of 3, if the sum of all the digits is divisible by 3, then the number is also divisible by 3. 3 + 7 + 9 + 1 + 7 + 2 + 5 + 6 + 4 + x + 8 must also be divisible by 3 ⇒ 52 + x 52 + 8 = 60 and 60 is divisible by 3. So, the largest possible value of x = 8.