help please

A four or five-digit PIN is to be selected from the ten digits 0,1,2,3,4,5,6,7,8,9.

2. Suppose any digit can be repeated, how many 4-digit PINs are possible if each PIN: a) must start with "7"? 7000 X b) must not end with "6"? 9000 c) must neither start with "3" nor end with "3"? 8100 3. Suppose no digit can be repeated, how many 5-digit PINs are possible if each PIN: a) must start with "7"? 15120 X b) must not end with "6"? 25200 X c) must neither start with "3" nor end with "3"? 30240 X