A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 6684678 can be evenly divided by 1 2 3 6 7 9 11 13 14 18 21 22 26 33 39 42 49 53 63 66 77 78 91 98 99 106 117 126 143 147 154 159 182 198 231 234 273 286 294 318 371 429 441 462 477 539 546 583 637 689 693 742 819 858 882 954 1,001 1,078 1,113 1,166 1,274 1,287 1,378 1,386 1,617 1,638 1,749 1,911 2,002 2,067 2,226 2,574 2,597 3,003 3,234 3,339 3,498 3,822 4,081 4,134 4,823 4,851 5,194 5,247 5,733 6,006 6,201 6,678 7,007 7,579 7,791 8,162 9,009 9,646 9,702 10,494 11,466 12,243 12,402 14,014 14,469 15,158 15,582 18,018 21,021 22,737 23,373 24,486 28,567 28,938 33,761 36,729 42,042 43,407 45,474 46,746 53,053 57,134 63,063 67,522 68,211 73,458 85,701 86,814 101,283 106,106 126,126 136,422 159,159 171,402 202,566 257,103 303,849 318,318 371,371 477,477 514,206 607,698 742,742 954,954 1,114,113 2,228,226 3,342,339 and 6,684,678, with no remainder.
Since 6,684,678 cannot be divided by just 1 and 6,684,678, it is not a prime number.