A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 2521680 can be evenly divided by 1 2 3 4 5 6 7 8 10 12 14 15 16 19 20 21 24 28 30 35 38 40 42 48 56 57 60 70 76 79 80 84 95 105 112 114 120 133 140 152 158 168 190 210 228 237 240 266 280 285 304 316 336 380 395 399 420 456 474 532 553 560 570 632 665 760 790 798 840 912 948 1,064 1,106 1,140 1,185 1,264 1,330 1,501 1,520 1,580 1,596 1,659 1,680 1,896 1,995 2,128 2,212 2,280 2,370 2,660 2,765 3,002 3,160 3,192 3,318 3,792 3,990 4,424 4,503 4,560 4,740 5,320 5,530 6,004 6,320 6,384 6,636 7,505 7,980 8,295 8,848 9,006 9,480 10,507 10,640 11,060 12,008 13,272 15,010 15,960 16,590 18,012 18,960 21,014 22,120 22,515 24,016 26,544 30,020 31,521 31,920 33,180 36,024 42,028 44,240 45,030 52,535 60,040 63,042 66,360 72,048 84,056 90,060 105,070 120,080 126,084 132,720 157,605 168,112 180,120 210,140 252,168 315,210 360,240 420,280 504,336 630,420 840,560 1,260,840 and 2,521,680, with no remainder.
Since 2,521,680 cannot be divided by just 1 and 2,521,680, it is not a prime number.