Hello, thank you for taking the time to post your question!
The key on this type of question is that you want to isolate the variable that you are solving for. In this case that means first subtracting 6 from both sides to combine like terms
6 + 3b = -18
3b = -24
Then dividing by 3 on both sides to get the “b” by itself
b = -24/3 = -8
I hope that helps you get moving in a better direction on this type of question! Feel free to reach out if you have any additional questions beyond that :)