A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1296420 can be evenly divided by 1 2 3 4 5 6 10 12 15 17 20 30 31 34 41 51 60 62 68 82 85 93 102 123 124 155 164 170 186 204 205 246 255 310 340 372 410 465 492 510 527 615 620 697 820 930 1,020 1,054 1,230 1,271 1,394 1,581 1,860 2,091 2,108 2,460 2,542 2,635 2,788 3,162 3,485 3,813 4,182 5,084 5,270 6,324 6,355 6,970 7,626 7,905 8,364 10,455 10,540 12,710 13,940 15,252 15,810 19,065 20,910 21,607 25,420 31,620 38,130 41,820 43,214 64,821 76,260 86,428 108,035 129,642 216,070 259,284 324,105 432,140 648,210 and 1,296,420, with no remainder.
Since 1,296,420 cannot be divided by just 1 and 1,296,420, it is not a prime number.