A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 420000 can be evenly divided by 1 2 3 4 5 6 7 8 10 12 14 15 16 20 21 24 25 28 30 32 35 40 42 48 50 56 60 70 75 80 84 96 100 105 112 120 125 140 150 160 168 175 200 210 224 240 250 280 300 336 350 375 400 420 480 500 525 560 600 625 672 700 750 800 840 875 1,000 1,050 1,120 1,200 1,250 1,400 1,500 1,680 1,750 1,875 2,000 2,100 2,400 2,500 2,625 2,800 3,000 3,360 3,500 3,750 4,000 4,200 4,375 5,000 5,250 5,600 6,000 7,000 7,500 8,400 8,750 10,000 10,500 12,000 13,125 14,000 15,000 16,800 17,500 20,000 21,000 26,250 28,000 30,000 35,000 42,000 52,500 60,000 70,000 84,000 105,000 140,000 210,000 and 420,000, with no remainder.
Since 420,000 cannot be divided by just 1 and 420,000, it is not a prime number.