"1. A z-score of zero always means  
a. 
the raw score does not exist.
 
b. 
the raw score exists, but is negligible.
 
c. 
the raw score almost never occurs.
 
d. 
the raw score is equal to the mean."

d.

Step-by-step explanation:

Here is an answer I found on the internet (kudos to investopedia.com)

If a Z-score is 0, it indicates that the data point's score is identical to the mean score.

In other words, it's saying that a z-score of zero has a standard deviation of zero.

x = -3

y = -18

step-by-step explanation:

to solve by substitution, substitute one variable and solve for the other. then substitute again to find the remaining variable.

x-1/3(3x-9) = -3 since y=3x-9 substittute y.

x-1/3(3x-9)=-3

x + x +3 = -3

2x+3=-3

2x+3-3= -3-3

2x=-6

x=-3

substitute again into y=3x-9

y=3(-3)-9

y=-9-9

y=-18