A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 6341874 can be evenly divided by 1 2 3 6 7 11 14 21 22 33 37 42 49 53 66 74 77 98 106 111 147 154 159 222 231 259 294 318 371 407 462 518 539 583 742 777 814 1,078 1,113 1,166 1,221 1,554 1,617 1,749 1,813 1,961 2,226 2,442 2,597 2,849 3,234 3,498 3,626 3,922 4,081 5,194 5,439 5,698 5,883 7,791 8,162 8,547 10,878 11,766 12,243 13,727 15,582 17,094 19,943 21,571 24,486 27,454 28,567 39,886 41,181 43,142 57,134 59,829 64,713 82,362 85,701 96,089 119,658 129,426 150,997 171,402 192,178 288,267 301,994 452,991 576,534 905,982 1,056,979 2,113,958 3,170,937 and 6,341,874, with no remainder.
Since 6,341,874 cannot be divided by just 1 and 6,341,874, it is not a prime number.