A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 2588400 can be evenly divided by 1 2 3 4 5 6 8 9 10 12 15 16 18 20 24 25 30 36 40 45 48 50 60 72 75 80 90 100 120 144 150 180 200 225 240 300 360 400 450 600 719 720 900 1,200 1,438 1,800 2,157 2,876 3,595 3,600 4,314 5,752 6,471 7,190 8,628 10,785 11,504 12,942 14,380 17,256 17,975 21,570 25,884 28,760 32,355 34,512 35,950 43,140 51,768 53,925 57,520 64,710 71,900 86,280 103,536 107,850 129,420 143,800 161,775 172,560 215,700 258,840 287,600 323,550 431,400 517,680 647,100 862,800 1,294,200 and 2,588,400, with no remainder.
Since 2,588,400 cannot be divided by just 1 and 2,588,400, it is not a prime number.