A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 2580660 can be evenly divided by 1 2 3 4 5 6 9 10 12 15 18 20 27 30 36 45 54 59 60 81 90 108 118 135 162 177 180 236 243 270 295 324 354 405 486 531 540 590 708 729 810 885 972 1,062 1,180 1,215 1,458 1,593 1,620 1,770 2,124 2,187 2,430 2,655 2,916 3,186 3,540 3,645 4,374 4,779 4,860 5,310 6,372 7,290 7,965 8,748 9,558 10,620 10,935 14,337 14,580 15,930 19,116 21,870 23,895 28,674 31,860 43,011 43,740 47,790 57,348 71,685 86,022 95,580 129,033 143,370 172,044 215,055 258,066 286,740 430,110 516,132 645,165 860,220 1,290,330 and 2,580,660, with no remainder.
Since 2,580,660 cannot be divided by just 1 and 2,580,660, it is not a prime number.