A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 636768 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 67 72 88 96 99 108 132 134 144 176 198 201 216 264 268 288 297 352 396 402 432 528 536 594 603 737 792 804 864 1,056 1,072 1,188 1,206 1,474 1,584 1,608 1,809 2,144 2,211 2,376 2,412 2,948 3,168 3,216 3,618 4,422 4,752 4,824 5,896 6,432 6,633 7,236 8,844 9,504 9,648 11,792 13,266 14,472 17,688 19,296 19,899 23,584 26,532 28,944 35,376 39,798 53,064 57,888 70,752 79,596 106,128 159,192 212,256 318,384 and 636,768, with no remainder.
Since 636,768 cannot be divided by just 1 and 636,768, it is not a prime number.