Question 337902
(-18 - 17= -35)
Sorry but the scaling doesn't hold when I send the solution but try to get the idea of a negative numbered line.

  -35       -30       -25      -20       -15       -10        -5         0    +3
 |-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|-|
   *                                   *      
 (-35)------------ -18 ------------- (-17) ----------- -17 ---------------
Starting from zero & going to the left on a numbered line 17 unite, you are at (-17). 
Going another 18 units to the left, you are at (-35).
-17+(-18)=-35 
17-18=-35