A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 858000 can be evenly divided by 1 2 3 4 5 6 8 10 11 12 13 15 16 20 22 24 25 26 30 33 39 40 44 48 50 52 55 60 65 66 75 78 80 88 100 104 110 120 125 130 132 143 150 156 165 176 195 200 208 220 240 250 260 264 275 286 300 312 325 330 375 390 400 429 440 500 520 528 550 572 600 624 650 660 715 750 780 825 858 880 975 1,000 1,040 1,100 1,144 1,200 1,300 1,320 1,375 1,430 1,500 1,560 1,625 1,650 1,716 1,950 2,000 2,145 2,200 2,288 2,600 2,640 2,750 2,860 3,000 3,120 3,250 3,300 3,432 3,575 3,900 4,125 4,290 4,400 4,875 5,200 5,500 5,720 6,000 6,500 6,600 6,864 7,150 7,800 8,250 8,580 9,750 10,725 11,000 11,440 13,000 13,200 14,300 15,600 16,500 17,160 17,875 19,500 21,450 22,000 26,000 28,600 33,000 34,320 35,750 39,000 42,900 53,625 57,200 66,000 71,500 78,000 85,800 107,250 143,000 171,600 214,500 286,000 429,000 and 858,000, with no remainder.
Since 858,000 cannot be divided by just 1 and 858,000, it is not a prime number.