A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 3205860 can be evenly divided by 1 2 3 4 5 6 7 10 12 14 15 17 20 21 28 30 34 35 42 51 60 68 70 84 85 102 105 119 140 170 204 210 238 255 340 357 420 449 476 510 595 714 898 1,020 1,190 1,347 1,428 1,785 1,796 2,245 2,380 2,694 3,143 3,570 4,490 5,388 6,286 6,735 7,140 7,633 8,980 9,429 12,572 13,470 15,266 15,715 18,858 22,899 26,940 30,532 31,430 37,716 38,165 45,798 47,145 53,431 62,860 76,330 91,596 94,290 106,862 114,495 152,660 160,293 188,580 213,724 228,990 267,155 320,586 457,980 534,310 641,172 801,465 1,068,620 1,602,930 and 3,205,860, with no remainder.
Since 3,205,860 cannot be divided by just 1 and 3,205,860, it is not a prime number.