A. write an expression for the area of the rectangle b. find the area for x=2

Hello! 
Your questions are quite simple:

A.
Area for rectangle [formula]: A=wl
So your answer is 3×(2x+5)

Explanation:
Since you only have a minimum amount of a number and variable all you have to do is multiply them both in order to get the area, however here we are only asked for an expression so basically, we are leaving out the answer and writing down the multiplication. 

B.
3×(2×2+5) = 27

Explanation:
I think the first explanation is very similar to this one, so I think you'll understand it.

Hope this helped!