A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 677376 can be evenly divided by 1 2 3 4 6 7 8 9 12 14 16 18 21 24 27 28 32 36 42 48 49 54 56 63 64 72 84 96 98 108 112 126 128 144 147 168 189 192 196 216 224 252 256 288 294 336 378 384 392 432 441 448 504 512 576 588 672 756 768 784 864 882 896 1,008 1,152 1,176 1,323 1,344 1,512 1,536 1,568 1,728 1,764 1,792 2,016 2,304 2,352 2,646 2,688 3,024 3,136 3,456 3,528 3,584 4,032 4,608 4,704 5,292 5,376 6,048 6,272 6,912 7,056 8,064 9,408 10,584 10,752 12,096 12,544 13,824 14,112 16,128 18,816 21,168 24,192 25,088 28,224 32,256 37,632 42,336 48,384 56,448 75,264 84,672 96,768 112,896 169,344 225,792 338,688 and 677,376, with no remainder.
Since 677,376 cannot be divided by just 1 and 677,376, it is not a prime number.