So to factorise an expression, you put it into a form where if you expanded the brackets, you'd get the given expression.. if that makes sense. Therefore, you need to multiply the 3 by something to get 18a, and so the first term is 6a. You don't need to multiply 3 by anything to get 3, so the second term is 1. The final result looks like:
3(6a + 1)