A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 7875000 can be evenly divided by 1 2 3 4 5 6 7 8 9 10 12 14 15 18 20 21 24 25 28 30 35 36 40 42 45 50 56 60 63 70 72 75 84 90 100 105 120 125 126 140 150 168 175 180 200 210 225 250 252 280 300 315 350 360 375 420 450 500 504 525 600 625 630 700 750 840 875 900 1,000 1,050 1,125 1,250 1,260 1,400 1,500 1,575 1,750 1,800 1,875 2,100 2,250 2,500 2,520 2,625 3,000 3,125 3,150 3,500 3,750 4,200 4,375 4,500 5,000 5,250 5,625 6,250 6,300 7,000 7,500 7,875 8,750 9,000 9,375 10,500 11,250 12,500 12,600 13,125 15,000 15,625 15,750 17,500 18,750 21,000 21,875 22,500 25,000 26,250 28,125 31,250 31,500 35,000 37,500 39,375 43,750 45,000 46,875 52,500 56,250 62,500 63,000 65,625 75,000 78,750 87,500 93,750 105,000 109,375 112,500 125,000 131,250 140,625 157,500 175,000 187,500 196,875 218,750 225,000 262,500 281,250 315,000 328,125 375,000 393,750 437,500 525,000 562,500 656,250 787,500 875,000 984,375 1,125,000 1,312,500 1,575,000 1,968,750 2,625,000 3,937,500 and 7,875,000, with no remainder.
Since 7,875,000 cannot be divided by just 1 and 7,875,000, it is not a prime number.