A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1289520 can be evenly divided by 1 2 3 4 5 6 8 9 10 12 15 16 18 20 24 27 30 36 40 45 48 54 60 72 80 81 90 108 120 135 144 162 180 199 216 240 270 324 360 398 405 432 540 597 648 720 796 810 995 1,080 1,194 1,296 1,592 1,620 1,791 1,990 2,160 2,388 2,985 3,184 3,240 3,582 3,980 4,776 5,373 5,970 6,480 7,164 7,960 8,955 9,552 10,746 11,940 14,328 15,920 16,119 17,910 21,492 23,880 26,865 28,656 32,238 35,820 42,984 47,760 53,730 64,476 71,640 80,595 85,968 107,460 128,952 143,280 161,190 214,920 257,904 322,380 429,840 644,760 and 1,289,520, with no remainder.
Since 1,289,520 cannot be divided by just 1 and 1,289,520, it is not a prime number.