A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 6655908 can be evenly divided by 1 2 3 4 6 7 12 14 17 21 28 34 42 51 59 68 79 84 102 118 119 158 177 204 236 237 238 316 354 357 413 474 476 553 708 714 826 948 1,003 1,106 1,239 1,343 1,428 1,652 1,659 2,006 2,212 2,478 2,686 3,009 3,318 4,012 4,029 4,661 4,956 5,372 6,018 6,636 7,021 8,058 9,322 9,401 12,036 13,983 14,042 16,116 18,644 18,802 21,063 27,966 28,084 28,203 32,627 37,604 42,126 55,932 56,406 65,254 79,237 84,252 97,881 112,812 130,508 158,474 195,762 237,711 316,948 391,524 475,422 554,659 950,844 1,109,318 1,663,977 2,218,636 3,327,954 and 6,655,908, with no remainder.
Since 6,655,908 cannot be divided by just 1 and 6,655,908, it is not a prime number.