A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1715850 can be evenly divided by 1 2 3 5 6 9 10 15 18 25 27 30 31 41 45 50 54 62 75 82 90 93 123 135 150 155 186 205 225 246 270 279 310 369 410 450 465 558 615 675 738 775 837 930 1,025 1,107 1,230 1,271 1,350 1,395 1,550 1,674 1,845 2,050 2,214 2,325 2,542 2,790 3,075 3,690 3,813 4,185 4,650 5,535 6,150 6,355 6,975 7,626 8,370 9,225 11,070 11,439 12,710 13,950 18,450 19,065 20,925 22,878 27,675 31,775 34,317 38,130 41,850 55,350 57,195 63,550 68,634 95,325 114,390 171,585 190,650 285,975 343,170 571,950 857,925 and 1,715,850, with no remainder.
Since 1,715,850 cannot be divided by just 1 and 1,715,850, it is not a prime number.