To solve the equation \(31 - 9x = 4\), we first isolate \(x\).
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Subtract 31 from both sides: \[ -9x = 4 - 31 \] \[ -9x = -27 \]
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Divide both sides by -9: \[ x = \frac{-27}{-9} \] \[ x = 3 \]
The solution set containing this single solution can be expressed in set notation as: \[ { 3 } \]
So the correct answer to the question is \({ 3 }\).