A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 8251320 can be evenly divided by 1 2 3 4 5 6 7 8 10 11 12 14 15 19 20 21 22 24 28 30 33 35 38 40 42 44 47 55 56 57 60 66 70 76 77 84 88 94 95 105 110 114 120 132 133 140 141 152 154 165 168 188 190 209 210 220 228 231 235 264 266 280 282 285 308 329 330 376 380 385 399 418 420 440 456 462 470 517 532 564 570 616 627 658 660 665 705 760 770 798 836 840 893 924 940 987 1,034 1,045 1,064 1,128 1,140 1,155 1,254 1,316 1,320 1,330 1,410 1,463 1,540 1,551 1,596 1,645 1,672 1,786 1,848 1,880 1,974 1,995 2,068 2,090 2,280 2,310 2,508 2,585 2,632 2,660 2,679 2,820 2,926 3,080 3,102 3,135 3,192 3,290 3,572 3,619 3,948 3,990 4,136 4,180 4,389 4,465 4,620 4,935 5,016 5,170 5,320 5,358 5,640 5,852 6,204 6,251 6,270 6,580 7,144 7,238 7,315 7,755 7,896 7,980 8,360 8,778 8,930 9,240 9,823 9,870 10,340 10,716 10,857 11,704 12,408 12,502 12,540 13,160 13,395 14,476 14,630 15,510 15,960 17,556 17,860 18,095 18,753 19,646 19,740 20,680 21,432 21,714 21,945 25,004 25,080 26,790 28,952 29,260 29,469 31,020 31,255 35,112 35,720 36,190 37,506 39,292 39,480 43,428 43,890 49,115 50,008 53,580 54,285 58,520 58,938 62,040 62,510 68,761 72,380 75,012 78,584 86,856 87,780 93,765 98,230 107,160 108,570 117,876 125,020 137,522 144,760 147,345 150,024 175,560 187,530 196,460 206,283 217,140 235,752 250,040 275,044 294,690 343,805 375,060 392,920 412,566 434,280 550,088 589,380 687,610 750,120 825,132 1,031,415 1,178,760 1,375,220 1,650,264 2,062,830 2,750,440 4,125,660 and 8,251,320, with no remainder.
Since 8,251,320 cannot be divided by just 1 and 8,251,320, it is not a prime number.