The sum of 3 times a number and 5 is 2 times the number plus 15. what is half the number?

Define x:
Let the sum be x 

Construct the equation:
The sum of 3 times a number and 5 is 2 times the number plus 15.
→ 3x + 5 = 2x + 15

Solve x:
3x + 5 = 2x + 15         | Equation
x + 5 = 15                   | Take away 2x from both sides
x = 10                         | Take away 5 from both sides

The number is 10.

8+(-2) =6
So the answer is 6