A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 780000 can be evenly divided by 1 2 3 4 5 6 8 10 12 13 15 16 20 24 25 26 30 32 39 40 48 50 52 60 65 75 78 80 96 100 104 120 125 130 150 156 160 195 200 208 240 250 260 300 312 325 375 390 400 416 480 500 520 600 624 625 650 750 780 800 975 1,000 1,040 1,200 1,248 1,250 1,300 1,500 1,560 1,625 1,875 1,950 2,000 2,080 2,400 2,500 2,600 3,000 3,120 3,250 3,750 3,900 4,000 4,875 5,000 5,200 6,000 6,240 6,500 7,500 7,800 8,125 9,750 10,000 10,400 12,000 13,000 15,000 15,600 16,250 19,500 20,000 24,375 26,000 30,000 31,200 32,500 39,000 48,750 52,000 60,000 65,000 78,000 97,500 130,000 156,000 195,000 260,000 390,000 and 780,000, with no remainder.
Since 780,000 cannot be divided by just 1 and 780,000, it is not a prime number.