1. The problem asks to write the phrase "13 subtracted from a number x" as a mathematical expression. 2. The phrase "13 subtracted from a number x" means we start with the number $x$ and subtract 13 from it. 3. The general form for subtraction is: $$\text{result} = \text{starting number} - \text{number to subtract}$$ 4. Applying this to our phrase, the expression is: $$x - 13$$ 5. This expression means take the number $x$ and subtract 13 from it. 6. Therefore, the expression for "13 subtracted from a number x" is $$x - 13$$.