A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1407840 can be evenly divided by 1 2 3 4 5 6 7 8 10 12 14 15 16 20 21 24 28 30 32 35 40 42 48 56 60 70 80 84 96 105 112 120 140 160 168 210 224 240 280 336 419 420 480 560 672 838 840 1,120 1,257 1,676 1,680 2,095 2,514 2,933 3,352 3,360 4,190 5,028 5,866 6,285 6,704 8,380 8,799 10,056 11,732 12,570 13,408 14,665 16,760 17,598 20,112 23,464 25,140 29,330 33,520 35,196 40,224 43,995 46,928 50,280 58,660 67,040 70,392 87,990 93,856 100,560 117,320 140,784 175,980 201,120 234,640 281,568 351,960 469,280 703,920 and 1,407,840, with no remainder.
Since 1,407,840 cannot be divided by just 1 and 1,407,840, it is not a prime number.