A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 6401376 can be evenly divided by 1 2 3 4 6 8 9 12 16 18 24 27 31 32 36 48 54 62 72 93 96 108 124 144 186 216 239 248 279 288 372 432 478 496 558 717 744 837 864 956 992 1,116 1,434 1,488 1,674 1,912 2,151 2,232 2,868 2,976 3,348 3,824 4,302 4,464 5,736 6,453 6,696 7,409 7,648 8,604 8,928 11,472 12,906 13,392 14,818 17,208 22,227 22,944 25,812 26,784 29,636 34,416 44,454 51,624 59,272 66,681 68,832 88,908 103,248 118,544 133,362 177,816 200,043 206,496 237,088 266,724 355,632 400,086 533,448 711,264 800,172 1,066,896 1,600,344 2,133,792 3,200,688 and 6,401,376, with no remainder.
Since 6,401,376 cannot be divided by just 1 and 6,401,376, it is not a prime number.