A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 6428688 can be evenly divided by 1 2 3 4 6 7 8 12 14 16 19 21 24 28 38 42 48 53 56 57 76 84 106 112 114 133 152 159 168 212 228 266 304 318 336 361 371 399 424 456 532 636 722 742 798 848 912 1,007 1,064 1,083 1,113 1,272 1,444 1,484 1,596 2,014 2,128 2,166 2,226 2,527 2,544 2,888 2,968 3,021 3,192 4,028 4,332 4,452 5,054 5,776 5,936 6,042 6,384 7,049 7,581 8,056 8,664 8,904 10,108 12,084 14,098 15,162 16,112 17,328 17,808 19,133 20,216 21,147 24,168 28,196 30,324 38,266 40,432 42,294 48,336 56,392 57,399 60,648 76,532 84,588 112,784 114,798 121,296 133,931 153,064 169,176 229,596 267,862 306,128 338,352 401,793 459,192 535,724 803,586 918,384 1,071,448 1,607,172 2,142,896 3,214,344 and 6,428,688, with no remainder.
Since 6,428,688 cannot be divided by just 1 and 6,428,688, it is not a prime number.