Well, whenever a number is next to a variable or next to parenthesis, (a variable is the letter[s] used to represent a value in the problem, such as 3b in this question), that means you would multiply the number times the letter, or the number times the answer in the parenthesis. So since we know a = 9, and b = 2, let's solve the equation now! :D
Now according to the order of operations of PEMDAS, let's solve what's in parenthesis first.
5(a + 3b)
⇒ a + 3b
⇒ a (a = 9), + 3 times b (b = 2)
⇒ 9 + 3 × 2
⇒ 9 + 6
⇒ 15.
⇒ 5(15)
⇒ 5 × 15
⇒ 75.
After doing all steps with the help of the order of operations of PEMDAS, your final answer would be 75. =)
I hope I helped! ^-^