A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 855000 can be evenly divided by 1 2 3 4 5 6 8 9 10 12 15 18 19 20 24 25 30 36 38 40 45 50 57 60 72 75 76 90 95 100 114 120 125 150 152 171 180 190 200 225 228 250 285 300 342 360 375 380 450 456 475 500 570 600 625 684 750 760 855 900 950 1,000 1,125 1,140 1,250 1,368 1,425 1,500 1,710 1,800 1,875 1,900 2,250 2,280 2,375 2,500 2,850 3,000 3,420 3,750 3,800 4,275 4,500 4,750 5,000 5,625 5,700 6,840 7,125 7,500 8,550 9,000 9,500 11,250 11,400 11,875 14,250 15,000 17,100 19,000 21,375 22,500 23,750 28,500 34,200 35,625 42,750 45,000 47,500 57,000 71,250 85,500 95,000 106,875 142,500 171,000 213,750 285,000 427,500 and 855,000, with no remainder.
Since 855,000 cannot be divided by just 1 and 855,000, it is not a prime number.