hello!
Use the Distributive Property, which states the following:-

where
we distribute a by multiplying it times b and c
In this case, we should distribute 9:-
9(3x-12)+9x
27x-108+9x
Combine Like Terms:-

note:-
Hope everything is clear; if you need any clarification/explanation, kindly let me know, and I will comment and/or edit my answer :)