The correct set notation for the solution set to the equation \( 31 - 9x = 4 \) is:
\({3}\)
This notation indicates that 3 is the single solution to the equation.
The other options do not properly represent set notation:
- \({x = 3}\) is not in standard set notation for a solution.
- \((x = 3)\) is not a set.
- \((3)\) suggests an ordered pair or just the number itself, which is also not standard set notation for a solution set.
So, the correct choice is \({3}\).