A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 2560536 can be evenly divided by 1 2 3 4 6 8 9 11 12 18 22 24 33 36 44 53 61 66 72 88 99 106 122 132 159 183 198 212 244 264 318 366 396 424 477 488 549 583 636 671 732 792 954 1,098 1,166 1,272 1,342 1,464 1,749 1,908 2,013 2,196 2,332 2,684 3,233 3,498 3,816 4,026 4,392 4,664 5,247 5,368 6,039 6,466 6,996 8,052 9,699 10,494 12,078 12,932 13,992 16,104 19,398 20,988 24,156 25,864 29,097 35,563 38,796 41,976 48,312 58,194 71,126 77,592 106,689 116,388 142,252 213,378 232,776 284,504 320,067 426,756 640,134 853,512 1,280,268 and 2,560,536, with no remainder.
Since 2,560,536 cannot be divided by just 1 and 2,560,536, it is not a prime number.