A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1366290 can be evenly divided by 1 2 3 5 6 9 10 15 17 18 19 30 34 38 45 47 51 57 85 90 94 95 102 114 141 153 170 171 190 235 255 282 285 306 323 342 423 470 510 570 646 705 765 799 846 855 893 969 1,410 1,530 1,598 1,615 1,710 1,786 1,938 2,115 2,397 2,679 2,907 3,230 3,995 4,230 4,465 4,794 4,845 5,358 5,814 7,191 7,990 8,037 8,930 9,690 11,985 13,395 14,382 14,535 15,181 16,074 23,970 26,790 29,070 30,362 35,955 40,185 45,543 71,910 75,905 80,370 91,086 136,629 151,810 227,715 273,258 455,430 683,145 and 1,366,290, with no remainder.
Since 1,366,290 cannot be divided by just 1 and 1,366,290, it is not a prime number.