The expression is 9*x.
Assuming that the phrase "doubled by 2" is tautological, the expression for a number multiplied by 2 (or a number doubled) is 2n.
the sum of a number and three
If there is an equal sign, it is called an equation; if there is a less-than, a less-than-or-equal, a greater-than, or a greater-than-or-equal sign, an inequality; if there is none of these, an expression.
an idiomatic expression
" 3x + 5 " does.