Question 7665
This is a real problem. First, get in the habit of thinking of numbers as points on a number line. The positive ones are to the right of zero. The negative ones are to the left of zero.
Evaluating an expression can be done by putting your finger at zero (you start with nothing) and then move right or left depending on the number. 
In this case you would going to the right 13 notches (one for each number). At that point you would go left 17 notches (-17). You can see that you go past zero on your way left, because it is only 13 notches to the left. In fact you go 4 past zero. Since you are left of zero by 4, the ending point (and the evaluation of the expression) is -4.
In general terms:
a plus added to a plus is a plus (both numbers are to the right)
a minus added to a minus is a minus (both numbers are to the left)
a plus added to a minus (we call that subtraction) depends on which number is "larger" in the sense of without a sign. That number's sign will be the sign of the answer.

(BTW, as an added bonus, the concept of which number is larger without the sign is called "absolute value" and is written |x|. No extra charge.)