A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 801780 can be evenly divided by 1 2 3 4 5 6 7 10 12 14 15 20 21 23 28 30 35 42 46 60 69 70 83 84 92 105 115 138 140 161 166 210 230 249 276 322 332 345 415 420 460 483 498 581 644 690 805 830 966 996 1,162 1,245 1,380 1,610 1,660 1,743 1,909 1,932 2,324 2,415 2,490 2,905 3,220 3,486 3,818 4,830 4,980 5,727 5,810 6,972 7,636 8,715 9,545 9,660 11,454 11,620 13,363 17,430 19,090 22,908 26,726 28,635 34,860 38,180 40,089 53,452 57,270 66,815 80,178 114,540 133,630 160,356 200,445 267,260 400,890 and 801,780, with no remainder.
Since 801,780 cannot be divided by just 1 and 801,780, it is not a prime number.