A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 328320 can be evenly divided by 1 2 3 4 5 6 8 9 10 12 15 16 18 19 20 24 27 30 32 36 38 40 45 48 54 57 60 64 72 76 80 90 95 96 108 114 120 128 135 144 152 160 171 180 190 192 216 228 240 270 285 288 304 320 342 360 380 384 432 456 480 513 540 570 576 608 640 684 720 760 855 864 912 960 1,026 1,080 1,140 1,152 1,216 1,368 1,440 1,520 1,710 1,728 1,824 1,920 2,052 2,160 2,280 2,432 2,565 2,736 2,880 3,040 3,420 3,456 3,648 4,104 4,320 4,560 5,130 5,472 5,760 6,080 6,840 7,296 8,208 8,640 9,120 10,260 10,944 12,160 13,680 16,416 17,280 18,240 20,520 21,888 27,360 32,832 36,480 41,040 54,720 65,664 82,080 109,440 164,160 and 328,320, with no remainder.
Since 328,320 cannot be divided by just 1 and 328,320, it is not a prime number.