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|5z + 1| = 21
|5z + 1-1| = 21-1

Remember if what you put is this sign: |x| and not this sign (x)
This sign, |x|, means absolute value. be careful in using that and do not confuse it with () parentheses.  
If you mean |5z+1|=21
Then, it can also mean that:
z=-4.4 is correct.
|-22+1| = 21
|-21| = 21, Correct, True