To make a two digit number think of there 
being two blanks to fill in, like this:

_ _


We can choose to fill in the 
first blank in these 3 ways

3 _
4 _
8 _

For each of those 3 ways to fill in the the first
digit, there are 3 ways to fill in the second blank:

For 3 _ the three ways to fill in the second blank
are:

3 3
3 4
3 8

For 4 _ the three ways to fill in the second blank
are:

4 3
4 4
4 8

For 8 _ the three ways to fill in the second blank
are:

8 3
8 4
8 8

That's why there are 3 times 3 or 9 ways to fill
in the two blanks.  3 ways for the first digit,
times 3 ways to fill in the second digit.

33, 34, 38, 43, 44, 48, 83, 84, 88

Edwin