A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 5120820 can be evenly divided by 1 2 3 4 5 6 9 10 12 15 18 20 27 29 30 36 45 54 58 60 81 87 90 108 109 116 135 145 162 174 180 218 261 270 290 324 327 348 405 435 436 522 540 545 580 654 783 810 870 981 1,044 1,090 1,305 1,308 1,566 1,620 1,635 1,740 1,962 2,180 2,349 2,610 2,943 3,132 3,161 3,270 3,915 3,924 4,698 4,905 5,220 5,886 6,322 6,540 7,830 8,829 9,396 9,483 9,810 11,745 11,772 12,644 14,715 15,660 15,805 17,658 18,966 19,620 23,490 28,449 29,430 31,610 35,316 37,932 44,145 46,980 47,415 56,898 58,860 63,220 85,347 88,290 94,830 113,796 142,245 170,694 176,580 189,660 256,041 284,490 341,388 426,735 512,082 568,980 853,470 1,024,164 1,280,205 1,706,940 2,560,410 and 5,120,820, with no remainder.
Since 5,120,820 cannot be divided by just 1 and 5,120,820, it is not a prime number.