A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1728480 can be evenly divided by 1 2 3 4 5 6 8 10 12 13 15 16 20 24 26 30 32 39 40 48 52 60 65 78 80 96 104 120 130 156 160 195 208 240 260 277 312 390 416 480 520 554 624 780 831 1,040 1,108 1,248 1,385 1,560 1,662 2,080 2,216 2,770 3,120 3,324 3,601 4,155 4,432 5,540 6,240 6,648 7,202 8,310 8,864 10,803 11,080 13,296 14,404 16,620 18,005 21,606 22,160 26,592 28,808 33,240 36,010 43,212 44,320 54,015 57,616 66,480 72,020 86,424 108,030 115,232 132,960 144,040 172,848 216,060 288,080 345,696 432,120 576,160 864,240 and 1,728,480, with no remainder.
Since 1,728,480 cannot be divided by just 1 and 1,728,480, it is not a prime number.