A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 6978720 can be evenly divided by 1 2 3 4 5 6 7 8 10 12 14 15 16 20 21 24 28 30 31 32 35 40 42 48 56 60 62 67 70 80 84 93 96 105 112 120 124 134 140 155 160 168 186 201 210 217 224 240 248 268 280 310 335 336 372 402 420 434 465 469 480 496 536 560 620 651 670 672 744 804 840 868 930 938 992 1,005 1,072 1,085 1,120 1,240 1,302 1,340 1,407 1,488 1,608 1,680 1,736 1,860 1,876 2,010 2,077 2,144 2,170 2,345 2,480 2,604 2,680 2,814 2,976 3,216 3,255 3,360 3,472 3,720 3,752 4,020 4,154 4,340 4,690 4,960 5,208 5,360 5,628 6,231 6,432 6,510 6,944 7,035 7,440 7,504 8,040 8,308 8,680 9,380 10,385 10,416 10,720 11,256 12,462 13,020 14,070 14,539 14,880 15,008 16,080 16,616 17,360 18,760 20,770 20,832 22,512 24,924 26,040 28,140 29,078 31,155 32,160 33,232 34,720 37,520 41,540 43,617 45,024 49,848 52,080 56,280 58,156 62,310 66,464 72,695 75,040 83,080 87,234 99,696 104,160 112,560 116,312 124,620 145,390 166,160 174,468 199,392 218,085 225,120 232,624 249,240 290,780 332,320 348,936 436,170 465,248 498,480 581,560 697,872 872,340 996,960 1,163,120 1,395,744 1,744,680 2,326,240 3,489,360 and 6,978,720, with no remainder.
Since 6,978,720 cannot be divided by just 1 and 6,978,720, it is not a prime number.