A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1378650 can be evenly divided by 1 2 3 5 6 7 10 13 14 15 21 25 26 30 35 39 42 50 65 70 75 78 91 101 105 130 150 175 182 195 202 210 273 303 325 350 390 455 505 525 546 606 650 707 910 975 1,010 1,050 1,313 1,365 1,414 1,515 1,950 2,121 2,275 2,525 2,626 2,730 3,030 3,535 3,939 4,242 4,550 5,050 6,565 6,825 7,070 7,575 7,878 9,191 10,605 13,130 13,650 15,150 17,675 18,382 19,695 21,210 27,573 32,825 35,350 39,390 45,955 53,025 55,146 65,650 91,910 98,475 106,050 137,865 196,950 229,775 275,730 459,550 689,325 and 1,378,650, with no remainder.
Since 1,378,650 cannot be divided by just 1 and 1,378,650, it is not a prime number.