In order to simplify this polynomial you have to add your common factors(common factors are number or variable or sometimes both that are the same) in this case 9a and 6a are common factors and 5b and 3b are common factors, so go ahead and add or subtract them.. They are all positives so this means you will only add, if you add 9a + 6a you get 15a and if you add 5b + 3b you get 8b so your final answer should be 15a+8b
Hope this helps