PLEASE HELP 15 points

Which is an equation for the nth terms of the sequence 12,15,18,21

At $1.32 million, they closed last month percent above what they paid in 2007." how much did they pay in 2007?

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.