Hi Ali,

I am going to call these strings of characters. You want strings of length 2, 3 and 4 and you have $10+26 = 36$ characters. Write each string starting with the leftmost character, which can't be zero so you have 35 choices for the first character, or in other words 35 one character strings.

Each of these one character string can be extended to a two character string by appending one character on the right. There are 36 characters to choose from so each one character string can be extended to 36 two character strings and hence there are $35 \times 36$ two character strings.

Each of these two character string can be extended to a three character string by appending one character on the right. How many three character strings are there?

Penny