Please post the whole question.

Your question can be rewritten as:
3(2a)+3(b)=135
2a+b=45 ; I'm pretty sure the question tells you positive multiples. Find out how many combinations satisfy that equation.

it's literally that. yes positives and integers.

even mult. of 3 is basically multiple of 6. So I just started pluggin numbers. Just wondering if theres even a quicker way.

I also tried 2a=3(45-b) but this took longer. 45-b must be even so b must be odd. etc..

Rule: Even + Odd = Odd

Given, Even multiple of 3 + a multiple of 3 = 135
Keep it simple
case 1 :126 (Even multiple of 3) + 9(a multiple of 3) = 135
case 2 :132(Even multiple of 3) + 3(a multiple of 3) = 135

So, there can be more than one combinations that add to 135. Hope this answers your question.

Just to clarify my point.

When you have 2a+b=45 and you need to find how many combinations of positive integers are there?

just divide the 45 by 2(coefficient in front of "a"), you will find 22 ways in which the equations can work.

a can be(1,2,..,21,22)

I hope I answered your question

damn, good point cywr.