A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 5100120 can be evenly divided by 1 2 3 4 5 6 8 9 10 12 15 18 20 24 30 31 36 40 45 60 62 72 90 93 120 124 155 180 186 248 279 310 360 372 457 465 558 620 744 914 930 1,116 1,240 1,371 1,395 1,828 1,860 2,232 2,285 2,742 2,790 3,656 3,720 4,113 4,570 5,484 5,580 6,855 8,226 9,140 10,968 11,160 13,710 14,167 16,452 18,280 20,565 27,420 28,334 32,904 41,130 42,501 54,840 56,668 70,835 82,260 85,002 113,336 127,503 141,670 164,520 170,004 212,505 255,006 283,340 340,008 425,010 510,012 566,680 637,515 850,020 1,020,024 1,275,030 1,700,040 2,550,060 and 5,100,120, with no remainder.
Since 5,100,120 cannot be divided by just 1 and 5,100,120, it is not a prime number.