A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1021800 can be evenly divided by 1 2 3 4 5 6 8 10 12 13 15 20 24 25 26 30 39 40 50 52 60 65 75 78 100 104 120 130 131 150 156 195 200 260 262 300 312 325 390 393 520 524 600 650 655 780 786 975 1,048 1,300 1,310 1,560 1,572 1,703 1,950 1,965 2,600 2,620 3,144 3,275 3,406 3,900 3,930 5,109 5,240 6,550 6,812 7,800 7,860 8,515 9,825 10,218 13,100 13,624 15,720 17,030 19,650 20,436 25,545 26,200 34,060 39,300 40,872 42,575 51,090 68,120 78,600 85,150 102,180 127,725 170,300 204,360 255,450 340,600 510,900 and 1,021,800, with no remainder.
Since 1,021,800 cannot be divided by just 1 and 1,021,800, it is not a prime number.