The answer is 3x+15
You multiply the x and 5 by 3 to get 3x and 15
The answers are:
1) 
2) 
3) 
The explanation for this answer is shown below.
1. To solve this problem you must apply the following proccedure:
2. You have the function 
3. You must switch
and
and then solve for
:

4. Finally, you obtain:

6 goes in the box, as it is being multiplied by the other numbers.
The horizontal line is refered to as a fraction bar or a vinculum. There are also other names, but these two are the ones I know the best.