A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1266300 can be evenly divided by 1 2 3 4 5 6 7 9 10 12 14 15 18 20 21 25 27 28 30 35 36 42 45 50 54 60 63 67 70 75 84 90 100 105 108 126 134 135 140 150 175 180 189 201 210 225 252 268 270 300 315 335 350 378 402 420 450 469 525 540 603 630 670 675 700 756 804 900 938 945 1,005 1,050 1,206 1,260 1,340 1,350 1,407 1,575 1,675 1,809 1,876 1,890 2,010 2,100 2,345 2,412 2,700 2,814 3,015 3,150 3,350 3,618 3,780 4,020 4,221 4,690 4,725 5,025 5,628 6,030 6,300 6,700 7,035 7,236 8,442 9,045 9,380 9,450 10,050 11,725 12,060 12,663 14,070 15,075 16,884 18,090 18,900 20,100 21,105 23,450 25,326 28,140 30,150 35,175 36,180 42,210 45,225 46,900 50,652 60,300 63,315 70,350 84,420 90,450 105,525 126,630 140,700 180,900 211,050 253,260 316,575 422,100 633,150 and 1,266,300, with no remainder.
Since 1,266,300 cannot be divided by just 1 and 1,266,300, it is not a prime number.