18x + 42y < 336
The boundary line is given by the function y =

In order to answer the question, you need to know what a multiple is. A multiple is a number which can be divided by the original number without a remainder. 
So for example multiples of 5 include: 5, 10, 15, 20, 25, etc... you are basically adding 5 each time. So in this question, you are being asked to list what the multiples of 9 are that are less than 80. You begin by listing the multiples of 9 and stop once you get to 80. 

9,18, 27, 36, 45, 54, 63, 72, 81. You stop here because the last multiple of 9 that you listed is more than 80. So your answer to this question would be: 
9,18, 27, 36, 45, 54, 63, 72.