A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 6858720 can be evenly divided by 1 2 3 4 5 6 8 9 10 11 12 15 16 18 20 22 24 30 32 33 36 40 44 45 48 55 60 66 72 80 88 90 96 99 110 120 132 144 160 165 176 180 198 220 240 264 288 330 352 360 396 433 440 480 495 528 660 720 792 866 880 990 1,056 1,299 1,320 1,440 1,584 1,732 1,760 1,980 2,165 2,598 2,640 3,168 3,464 3,897 3,960 4,330 4,763 5,196 5,280 6,495 6,928 7,794 7,920 8,660 9,526 10,392 12,990 13,856 14,289 15,588 15,840 17,320 19,052 19,485 20,784 23,815 25,980 28,578 31,176 34,640 38,104 38,970 41,568 42,867 47,630 51,960 57,156 62,352 69,280 71,445 76,208 77,940 85,734 95,260 103,920 114,312 124,704 142,890 152,416 155,880 171,468 190,520 207,840 214,335 228,624 285,780 311,760 342,936 381,040 428,670 457,248 571,560 623,520 685,872 762,080 857,340 1,143,120 1,371,744 1,714,680 2,286,240 3,429,360 and 6,858,720, with no remainder.
Since 6,858,720 cannot be divided by just 1 and 6,858,720, it is not a prime number.