the solution set
An algebraic equation or inequality can have a solution, an algebraic expression cannot. If substituting a number in place of a variable results in the equation or inequality being a true statement, then that number is a solution of the equation or inequality.
What is the solution to the expression? 3-4(1/2)+7
A true solution doesn't contain any insoluble matter and has only one phase.
x - 9 is an expression, neither an equation not an inequality. An expression cannot have a solution.
2x + 4 is an expression, not an equation. There can be no solution set for an expression.