A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 2811780 can be evenly divided by 1 2 3 4 5 6 9 10 12 15 18 20 27 30 36 41 45 54 60 82 90 108 123 127 135 164 180 205 246 254 270 369 381 410 492 508 540 615 635 738 762 820 1,107 1,143 1,230 1,270 1,476 1,524 1,845 1,905 2,214 2,286 2,460 2,540 3,429 3,690 3,810 4,428 4,572 5,207 5,535 5,715 6,858 7,380 7,620 10,414 11,070 11,430 13,716 15,621 17,145 20,828 22,140 22,860 26,035 31,242 34,290 46,863 52,070 62,484 68,580 78,105 93,726 104,140 140,589 156,210 187,452 234,315 281,178 312,420 468,630 562,356 702,945 937,260 1,405,890 and 2,811,780, with no remainder.
Since 2,811,780 cannot be divided by just 1 and 2,811,780, it is not a prime number.