A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 6588400 can be evenly divided by 1 2 4 5 7 8 10 13 14 16 20 25 26 28 35 40 50 52 56 65 70 80 91 100 104 112 130 140 175 181 182 200 208 260 280 325 350 362 364 400 455 520 560 650 700 724 728 905 910 1,040 1,267 1,300 1,400 1,448 1,456 1,810 1,820 2,275 2,353 2,534 2,600 2,800 2,896 3,620 3,640 4,525 4,550 4,706 5,068 5,200 6,335 7,240 7,280 9,050 9,100 9,412 10,136 11,765 12,670 14,480 16,471 18,100 18,200 18,824 20,272 23,530 25,340 31,675 32,942 36,200 36,400 37,648 47,060 50,680 58,825 63,350 65,884 72,400 82,355 94,120 101,360 117,650 126,700 131,768 164,710 188,240 235,300 253,400 263,536 329,420 411,775 470,600 506,800 658,840 823,550 941,200 1,317,680 1,647,100 3,294,200 and 6,588,400, with no remainder.
Since 6,588,400 cannot be divided by just 1 and 6,588,400, it is not a prime number.