A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 800640 can be evenly divided by 1 2 3 4 5 6 8 9 10 12 15 16 18 20 24 30 32 36 40 45 48 60 64 72 80 90 96 120 128 139 144 160 180 192 240 278 288 320 360 384 417 480 556 576 640 695 720 834 960 1,112 1,152 1,251 1,390 1,440 1,668 1,920 2,085 2,224 2,502 2,780 2,880 3,336 4,170 4,448 5,004 5,560 5,760 6,255 6,672 8,340 8,896 10,008 11,120 12,510 13,344 16,680 17,792 20,016 22,240 25,020 26,688 33,360 40,032 44,480 50,040 53,376 66,720 80,064 88,960 100,080 133,440 160,128 200,160 266,880 400,320 and 800,640, with no remainder.
Since 800,640 cannot be divided by just 1 and 800,640, it is not a prime number.