A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 590400 can be evenly divided by 1 2 3 4 5 6 8 9 10 12 15 16 18 20 24 25 30 32 36 40 41 45 48 50 60 64 72 75 80 82 90 96 100 120 123 144 150 160 164 180 192 200 205 225 240 246 288 300 320 328 360 369 400 410 450 480 492 576 600 615 656 720 738 800 820 900 960 984 1,025 1,200 1,230 1,312 1,440 1,476 1,600 1,640 1,800 1,845 1,968 2,050 2,400 2,460 2,624 2,880 2,952 3,075 3,280 3,600 3,690 3,936 4,100 4,800 4,920 5,904 6,150 6,560 7,200 7,380 7,872 8,200 9,225 9,840 11,808 12,300 13,120 14,400 14,760 16,400 18,450 19,680 23,616 24,600 29,520 32,800 36,900 39,360 49,200 59,040 65,600 73,800 98,400 118,080 147,600 196,800 295,200 and 590,400, with no remainder.
Since 590,400 cannot be divided by just 1 and 590,400, it is not a prime number.