A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1878912 can be evenly divided by 1 2 3 4 6 7 8 9 12 14 16 18 21 24 28 32 36 42 48 56 63 64 72 84 96 112 126 128 144 168 192 224 233 252 288 336 384 448 466 504 576 672 699 896 932 1,008 1,152 1,344 1,398 1,631 1,864 2,016 2,097 2,688 2,796 3,262 3,728 4,032 4,194 4,893 5,592 6,524 7,456 8,064 8,388 9,786 11,184 13,048 14,679 14,912 16,776 19,572 22,368 26,096 29,358 29,824 33,552 39,144 44,736 52,192 58,716 67,104 78,288 89,472 104,384 117,432 134,208 156,576 208,768 234,864 268,416 313,152 469,728 626,304 939,456 and 1,878,912, with no remainder.
Since 1,878,912 cannot be divided by just 1 and 1,878,912, it is not a prime number.