z - (-9) is the same as z + 9. Subtracting a negative is the same as adding.
Therefore 17 = z - (-9) is the same as 17 = z + 9
To get 'z' all by itself, we need to subtract 9 from both sides. We subtract 9 from both sides to undo the "plus 9" occurring to the 'z' on the right side.
17 = z + 9 17-9 = z + 9=9 ... subtracting 9 from both sides 8 = z z = 8
So the answer is z = 8
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Check:
17 = z - (-9) 17 = 8 - (-9) ... replace z with 8 17 = 8 + 9 17 = 17 Answer checks out
