cases: 73 is in the front, middle or end.
case where it is the front:
first - second digit, 1 choice each
3rd digit: 1~9 = 9
4th : 9
case where it is the middle:
first digit has 9 choices, and the 4th also has 9 choices. 2nd and 3rd have one choice.
case where it is the end.
same thing, first digit 9 choices, 2nd digit 9 choices, 3 and 4, 1 choice.
So! 9 times 9+9 times 9+9 times 9 = ill leave that to you