How do you write this as a variable expression “The sum of $ - 3$ and a number”?

Hint:Algebraic operation or term for the phrase “the sum” is “+” that is additional operation and the phrase “a number” means it can be any digit or decimal or is generally say a real number so consider it as a variable. Use this information to express the given phrase as an algebraic variable expression.

Complete step by step answer:
In order to express the phrase “The sum of $ - 3$ and a number”, as an algebraic variable expression, we should have the information about what types of algebraic operations are taking place in this phrase. From the given phrase,
“The sum of $ - 3$ and a number”
We can see that there is only one algebraic operation taking place, but what it actually means in algebraic significance.Let us see some general phrases and their algebraic expressions in order to express the given phrase.
Sum of “a” and “b” represents the algebraic expression: $a + b$
The second term “a number” can be any number belonging to the set of real numbers so we will take its value in an alphabet (say $x$) then we can easily express the given phrase.
With the help of above information expressing the given phrase into algebraic variable expression as,
“The sum of $ - 3$ and a number” $ = - 3 + x = x - 3$

Note:The commutative property is applicable in multiplication and additional operation in algebra that’s why we have written $ - 3 + x = x - 3$ above. Also note that this property is not applicable for division and subtraction.