Helps me solve this problem please 12 and 14

Add the two expressions 3.8b - 7 and 2.9b + 9 enter your answer in simplified form

Ascientist wants to use a model to present the results of his detailed scientific investigation. why would a model be useful? a )because the model makes the concepts easier to understand b) because the model is easy to put together and to use c) because the model prevents other scientists from asking questions d) because the model requires the audience to pay full attention to it i don't know what to choose because between a and d a is much more likely than d because it only wants to get the attention of the audience. i'm confuse.

Hi, can anyone show me how to do this problem? 100 points for this. in advance