A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1284000 can be evenly divided by 1 2 3 4 5 6 8 10 12 15 16 20 24 25 30 32 40 48 50 60 75 80 96 100 107 120 125 150 160 200 214 240 250 300 321 375 400 428 480 500 535 600 642 750 800 856 1,000 1,070 1,200 1,284 1,500 1,605 1,712 2,000 2,140 2,400 2,568 2,675 3,000 3,210 3,424 4,000 4,280 5,136 5,350 6,000 6,420 8,025 8,560 10,272 10,700 12,000 12,840 13,375 16,050 17,120 21,400 25,680 26,750 32,100 40,125 42,800 51,360 53,500 64,200 80,250 85,600 107,000 128,400 160,500 214,000 256,800 321,000 428,000 642,000 and 1,284,000, with no remainder.
Since 1,284,000 cannot be divided by just 1 and 1,284,000, it is not a prime number.