A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 3959550 can be evenly divided by 1 2 3 5 6 7 9 10 14 15 18 21 25 27 30 35 42 45 50 54 63 70 75 90 105 126 135 150 175 189 210 225 270 315 350 378 419 450 525 630 675 838 945 1,050 1,257 1,350 1,575 1,890 2,095 2,514 2,933 3,150 3,771 4,190 4,725 5,866 6,285 7,542 8,799 9,450 10,475 11,313 12,570 14,665 17,598 18,855 20,950 22,626 26,397 29,330 31,425 37,710 43,995 52,794 56,565 62,850 73,325 79,191 87,990 94,275 113,130 131,985 146,650 158,382 188,550 219,975 263,970 282,825 395,955 439,950 565,650 659,925 791,910 1,319,850 1,979,775 and 3,959,550, with no remainder.
Since 3,959,550 cannot be divided by just 1 and 3,959,550, it is not a prime number.