A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 7131780 can be evenly divided by 1 2 3 4 5 6 9 10 12 15 18 20 27 30 36 45 47 54 60 90 94 108 135 141 180 188 235 270 281 282 423 470 540 562 564 705 843 846 940 1,124 1,269 1,405 1,410 1,686 1,692 2,115 2,529 2,538 2,810 2,820 3,372 4,215 4,230 5,058 5,076 5,620 6,345 7,587 8,430 8,460 10,116 12,645 12,690 13,207 15,174 16,860 25,290 25,380 26,414 30,348 37,935 39,621 50,580 52,828 66,035 75,870 79,242 118,863 132,070 151,740 158,484 198,105 237,726 264,140 356,589 396,210 475,452 594,315 713,178 792,420 1,188,630 1,426,356 1,782,945 2,377,260 3,565,890 and 7,131,780, with no remainder.
Since 7,131,780 cannot be divided by just 1 and 7,131,780, it is not a prime number.