A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 7851060 can be evenly divided by 1 2 3 4 5 6 7 9 10 12 14 15 18 20 21 27 28 30 31 35 36 42 45 54 60 62 63 67 70 84 90 93 105 108 124 126 134 135 140 155 180 186 189 201 210 217 252 268 270 279 310 315 335 372 378 402 420 434 465 469 540 558 603 620 630 651 670 756 804 837 868 930 938 945 1,005 1,085 1,116 1,206 1,260 1,302 1,340 1,395 1,407 1,674 1,809 1,860 1,876 1,890 1,953 2,010 2,077 2,170 2,345 2,412 2,604 2,790 2,814 3,015 3,255 3,348 3,618 3,780 3,906 4,020 4,154 4,185 4,221 4,340 4,690 5,580 5,628 5,859 6,030 6,231 6,510 7,035 7,236 7,812 8,308 8,370 8,442 9,045 9,380 9,765 10,385 11,718 12,060 12,462 12,663 13,020 14,070 14,539 16,740 16,884 18,090 18,693 19,530 20,770 21,105 23,436 24,924 25,326 28,140 29,078 29,295 31,155 36,180 37,386 39,060 41,540 42,210 43,617 50,652 56,079 58,156 58,590 62,310 63,315 72,695 74,772 84,420 87,234 93,465 112,158 117,180 124,620 126,630 130,851 145,390 174,468 186,930 218,085 224,316 253,260 261,702 280,395 290,780 373,860 392,553 436,170 523,404 560,790 654,255 785,106 872,340 1,121,580 1,308,510 1,570,212 1,962,765 2,617,020 3,925,530 and 7,851,060, with no remainder.
Since 7,851,060 cannot be divided by just 1 and 7,851,060, it is not a prime number.