The correct set notation to express the solution set to the equation \(31 - 9x = 4\) is:
\({x = 3}\) or \({3}\).
Either \({3}\) or \({x = 3}\) would generally be acceptable, but the most common representation for a solution set is \({3}\).
So if you have to choose from the options provided, the most appropriate would be:
\({3}\) or \({x = 3}\) (depending on the options given).