A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1557920 can be evenly divided by 1 2 4 5 7 8 10 13 14 16 20 26 28 32 35 40 52 56 65 70 80 91 104 107 112 130 140 160 182 208 214 224 260 280 364 416 428 455 520 535 560 728 749 856 910 1,040 1,070 1,120 1,391 1,456 1,498 1,712 1,820 2,080 2,140 2,782 2,912 2,996 3,424 3,640 3,745 4,280 5,564 5,992 6,955 7,280 7,490 8,560 9,737 11,128 11,984 13,910 14,560 14,980 17,120 19,474 22,256 23,968 27,820 29,960 38,948 44,512 48,685 55,640 59,920 77,896 97,370 111,280 119,840 155,792 194,740 222,560 311,584 389,480 778,960 and 1,557,920, with no remainder.
Since 1,557,920 cannot be divided by just 1 and 1,557,920, it is not a prime number.