A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 7768728 can be evenly divided by 1 2 3 4 6 8 9 11 12 17 18 22 24 33 34 36 44 51 66 68 72 88 99 102 132 136 153 187 198 204 264 306 374 396 408 561 577 612 748 792 1,122 1,154 1,224 1,496 1,683 1,731 2,244 2,308 3,366 3,462 4,488 4,616 5,193 6,347 6,732 6,924 9,809 10,386 12,694 13,464 13,848 19,041 19,618 20,772 25,388 29,427 38,082 39,236 41,544 50,776 57,123 58,854 76,164 78,472 88,281 107,899 114,246 117,708 152,328 176,562 215,798 228,492 235,416 323,697 353,124 431,596 456,984 647,394 706,248 863,192 971,091 1,294,788 1,942,182 2,589,576 3,884,364 and 7,768,728, with no remainder.
Since 7,768,728 cannot be divided by just 1 and 7,768,728, it is not a prime number.