A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 978912 can be evenly divided by 1 2 3 4 6 8 9 11 12 16 18 22 24 27 32 33 36 44 48 54 66 72 88 96 99 103 108 132 144 176 198 206 216 264 288 297 309 352 396 412 432 528 594 618 792 824 864 927 1,056 1,133 1,188 1,236 1,584 1,648 1,854 2,266 2,376 2,472 2,781 3,168 3,296 3,399 3,708 4,532 4,752 4,944 5,562 6,798 7,416 9,064 9,504 9,888 10,197 11,124 13,596 14,832 18,128 20,394 22,248 27,192 29,664 30,591 36,256 40,788 44,496 54,384 61,182 81,576 88,992 108,768 122,364 163,152 244,728 326,304 489,456 and 978,912, with no remainder.
Since 978,912 cannot be divided by just 1 and 978,912, it is not a prime number.