Question 211251
3/(7z+5) 
Find all numbers for which the rational expression is undefined 
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It might have two answers if the denominator is a quadratic.
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But yours has 7z+5 which cannot be zero.
So you solve it for zero: 
7z+5=0
z = -5/7
That is the value z cannot have.
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Cheers,
Stan H.