A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 5266404 can be evenly divided by 1 2 3 4 6 9 11 12 13 18 22 26 27 31 33 36 39 44 52 54 62 66 78 93 99 108 117 121 124 132 143 156 186 198 234 242 279 286 297 341 351 363 372 396 403 429 468 484 558 572 594 682 702 726 806 837 858 1,023 1,089 1,116 1,188 1,209 1,287 1,364 1,404 1,452 1,573 1,612 1,674 1,716 2,046 2,178 2,418 2,574 3,069 3,146 3,267 3,348 3,627 3,751 3,861 4,092 4,356 4,433 4,719 4,836 5,148 6,138 6,292 6,534 7,254 7,502 7,722 8,866 9,207 9,438 10,881 11,253 12,276 13,068 13,299 14,157 14,508 15,004 15,444 17,732 18,414 18,876 21,762 22,506 26,598 28,314 33,759 36,828 39,897 42,471 43,524 45,012 48,763 53,196 56,628 67,518 79,794 84,942 97,526 101,277 119,691 135,036 146,289 159,588 169,884 195,052 202,554 239,382 292,578 405,108 438,867 478,764 585,156 877,734 1,316,601 1,755,468 2,633,202 and 5,266,404, with no remainder.
Since 5,266,404 cannot be divided by just 1 and 5,266,404, it is not a prime number.