To solve the equation \( 31 - 9x = 4 \), we need to isolate \( x \).
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Start by subtracting 31 from both sides: \[ -9x = 4 - 31 \] \[ -9x = -27 \]
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Next, divide both sides by -9: \[ x = \frac{-27}{-9} \] \[ x = 3 \]
The solution set contains the single solution \( x = 3 \). In set notation, this can be expressed as:
\[ {3} \]
Thus, the correct notation to express the solution set of the equation \( 31 - 9x = 4 \) is \( {3} \).