A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 8390520 can be evenly divided by 1 2 3 4 5 6 8 9 10 12 15 17 18 20 24 27 30 34 36 40 45 51 54 60 68 72 85 90 102 108 120 135 136 153 170 180 204 216 255 270 306 340 360 408 457 459 510 540 612 680 765 914 918 1,020 1,080 1,224 1,371 1,530 1,828 1,836 2,040 2,285 2,295 2,742 3,060 3,656 3,672 4,113 4,570 4,590 5,484 6,120 6,855 7,769 8,226 9,140 9,180 10,968 12,339 13,710 15,538 16,452 18,280 18,360 20,565 23,307 24,678 27,420 31,076 32,904 38,845 41,130 46,614 49,356 54,840 61,695 62,152 69,921 77,690 82,260 93,228 98,712 116,535 123,390 139,842 155,380 164,520 186,456 209,763 233,070 246,780 279,684 310,760 349,605 419,526 466,140 493,560 559,368 699,210 839,052 932,280 1,048,815 1,398,420 1,678,104 2,097,630 2,796,840 4,195,260 and 8,390,520, with no remainder.
Since 8,390,520 cannot be divided by just 1 and 8,390,520, it is not a prime number.