A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 6721800 can be evenly divided by 1 2 3 4 5 6 8 10 12 15 17 20 24 25 30 34 40 50 51 60 68 75 85 100 102 120 136 150 170 200 204 255 300 340 408 425 510 600 659 680 850 1,020 1,275 1,318 1,700 1,977 2,040 2,550 2,636 3,295 3,400 3,954 5,100 5,272 6,590 7,908 9,885 10,200 11,203 13,180 15,816 16,475 19,770 22,406 26,360 32,950 33,609 39,540 44,812 49,425 56,015 65,900 67,218 79,080 89,624 98,850 112,030 131,800 134,436 168,045 197,700 224,060 268,872 280,075 336,090 395,400 448,120 560,150 672,180 840,225 1,120,300 1,344,360 1,680,450 2,240,600 3,360,900 and 6,721,800, with no remainder.
Since 6,721,800 cannot be divided by just 1 and 6,721,800, it is not a prime number.