A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 2539768 can be evenly divided by 1 2 4 7 8 11 14 19 22 28 31 38 44 49 56 62 76 77 88 98 124 133 152 154 196 209 217 248 266 308 341 392 418 434 532 539 589 616 682 836 868 931 1,064 1,078 1,178 1,364 1,463 1,519 1,672 1,736 1,862 2,156 2,356 2,387 2,728 2,926 3,038 3,724 4,123 4,312 4,712 4,774 5,852 6,076 6,479 7,448 8,246 9,548 10,241 11,704 12,152 12,958 16,492 16,709 19,096 20,482 25,916 28,861 32,984 33,418 40,964 45,353 51,832 57,722 66,836 81,928 90,706 115,444 133,672 181,412 230,888 317,471 362,824 634,942 1,269,884 and 2,539,768, with no remainder.
Since 2,539,768 cannot be divided by just 1 and 2,539,768, it is not a prime number.