The phrase “the sum of three and a number” can be modeled by the expression \( 3 + x \), where \( x \) represents the unknown number. Given your options, none of them directly match \( 3 + x \).
However, if we align with the provided responses, the closest equivalent would be \( 3 + x \) if it were there, but since none are listed, the correct response isn't present among them.
If we had to choose based on what seems to fit the closest among given answers, we should note that:
- \( 3 + x \) would directly represent the sum of three and a number.
- \( 3x \) incorrectly implies multiplication, not addition.
- \( 3 - x \) denotes subtraction, not a sum.
- \( x - 3 \) denotes subtraction, not a sum.
- \( x \cdot 3 \) is multiplication, not addition.
Therefore, based on your responses, the representation isn't adequately provided in your options, as the correct expression for “the sum of three and a number” is \( 3 + x \).