A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1721400 can be evenly divided by 1 2 3 4 5 6 8 10 12 15 19 20 24 25 30 38 40 50 57 60 75 76 95 100 114 120 150 151 152 190 200 228 285 300 302 380 453 456 475 570 600 604 755 760 906 950 1,140 1,208 1,425 1,510 1,812 1,900 2,265 2,280 2,850 2,869 3,020 3,624 3,775 3,800 4,530 5,700 5,738 6,040 7,550 8,607 9,060 11,325 11,400 11,476 14,345 15,100 17,214 18,120 22,650 22,952 28,690 30,200 34,428 43,035 45,300 57,380 68,856 71,725 86,070 90,600 114,760 143,450 172,140 215,175 286,900 344,280 430,350 573,800 860,700 and 1,721,400, with no remainder.
Since 1,721,400 cannot be divided by just 1 and 1,721,400, it is not a prime number.