Question 907688
<pre> 
  _________________________________________________
  V   |   |   W   |   |   X   |   |   Y   |   |   Z   
  ?           ?           ?           ?           ?
 
Since W+Z = 0, Z=-W. Z is the negative (opposite) of Z.
So Z must be as far to the right of 0 as Z is to the left 
of 0.  That is, 0 must be at the midpoint between W and Z.

We put 0 at the midpoint between W and Z, and we have

  _________________________________________________
  V   |   |   W   |   |   X   | 0 |   Y   |   |   Z  
  ?           ?           ?     0     ?           ?


Since W and Z are 9 units apart, and half of 9 is 4.5,
W must be at -4.5 and Z at +4.5.  So we have

  _________________________________________________
  V   |   |   W   |   |   X   | 0 |   Y   |   |   Z 
  ?         -4.5          ?     0     ?          4.5   
 
Then we can fill in for the other question marks, since
they are all spaced 3 between each two adjacent letters:

  _________________________________________________
  V   |   |   W   |   |   X   | 0 |   Y   |   |   Z 
-7.5        -4.5        -1.5    0    1.5         4.5

The answer is -7.5, choice a.

Edwin</pre>