A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 390000 can be evenly divided by 1 2 3 4 5 6 8 10 12 13 15 16 20 24 25 26 30 39 40 48 50 52 60 65 75 78 80 100 104 120 125 130 150 156 195 200 208 240 250 260 300 312 325 375 390 400 500 520 600 624 625 650 750 780 975 1,000 1,040 1,200 1,250 1,300 1,500 1,560 1,625 1,875 1,950 2,000 2,500 2,600 3,000 3,120 3,250 3,750 3,900 4,875 5,000 5,200 6,000 6,500 7,500 7,800 8,125 9,750 10,000 13,000 15,000 15,600 16,250 19,500 24,375 26,000 30,000 32,500 39,000 48,750 65,000 78,000 97,500 130,000 195,000 and 390,000, with no remainder.
Since 390,000 cannot be divided by just 1 and 390,000, it is not a prime number.