A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 6291870 can be evenly divided by 1 2 3 5 6 10 13 15 17 26 30 34 39 51 65 73 78 85 102 130 146 169 170 195 219 221 255 338 365 390 438 442 507 510 663 730 845 949 1,014 1,095 1,105 1,241 1,326 1,690 1,898 2,190 2,210 2,482 2,535 2,847 2,873 3,315 3,723 4,745 5,070 5,694 5,746 6,205 6,630 7,446 8,619 9,490 12,337 12,410 14,235 14,365 16,133 17,238 18,615 24,674 28,470 28,730 32,266 37,011 37,230 43,095 48,399 61,685 74,022 80,665 86,190 96,798 123,370 161,330 185,055 209,729 241,995 370,110 419,458 483,990 629,187 1,048,645 1,258,374 2,097,290 3,145,935 and 6,291,870, with no remainder.
Since 6,291,870 cannot be divided by just 1 and 6,291,870, it is not a prime number.