A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 287040 can be evenly divided by 1 2 3 4 5 6 8 10 12 13 15 16 20 23 24 26 30 32 39 40 46 48 52 60 64 65 69 78 80 92 96 104 115 120 130 138 156 160 184 192 195 208 230 240 260 276 299 312 320 345 368 390 416 460 480 520 552 598 624 690 736 780 832 897 920 960 1,040 1,104 1,196 1,248 1,380 1,472 1,495 1,560 1,794 1,840 2,080 2,208 2,392 2,496 2,760 2,990 3,120 3,588 3,680 4,160 4,416 4,485 4,784 5,520 5,980 6,240 7,176 7,360 8,970 9,568 11,040 11,960 12,480 14,352 17,940 19,136 22,080 23,920 28,704 35,880 47,840 57,408 71,760 95,680 143,520 and 287,040, with no remainder.
Since 287,040 cannot be divided by just 1 and 287,040, it is not a prime number.