To determine this we need to determine what ^x/_z equals.  We need to eliminate the y variable and we can do this by subtracting out the two y's.  

Subtracting
-x + y + 3z = 5
-
(x  + y + -3z = 5)
-2x + 6z = 0

2x = 6z => now we can divide both sides by 2:

x = ^6z/₂ => now we can divide both sides by z:

^x/_z = ⁶/₂ = 3

Now we know that ^x/_z is equal to 3. 

The correct answer is D.