A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1620000 can be evenly divided by 1 2 3 4 5 6 8 9 10 12 15 16 18 20 24 25 27 30 32 36 40 45 48 50 54 60 72 75 80 81 90 96 100 108 120 125 135 144 150 160 162 180 200 216 225 240 250 270 288 300 324 360 375 400 405 432 450 480 500 540 600 625 648 675 720 750 800 810 864 900 1,000 1,080 1,125 1,200 1,250 1,296 1,350 1,440 1,500 1,620 1,800 1,875 2,000 2,025 2,160 2,250 2,400 2,500 2,592 2,700 3,000 3,240 3,375 3,600 3,750 4,000 4,050 4,320 4,500 5,000 5,400 5,625 6,000 6,480 6,750 7,200 7,500 8,100 9,000 10,000 10,125 10,800 11,250 12,000 12,960 13,500 15,000 16,200 16,875 18,000 20,000 20,250 21,600 22,500 27,000 30,000 32,400 33,750 36,000 40,500 45,000 50,625 54,000 60,000 64,800 67,500 81,000 90,000 101,250 108,000 135,000 162,000 180,000 202,500 270,000 324,000 405,000 540,000 810,000 and 1,620,000, with no remainder.
Since 1,620,000 cannot be divided by just 1 and 1,620,000, it is not a prime number.