How do you define a variable and write an expression for each phrase: the sum of eighteen and b?
You will need to define a variable when a question refers to "the number of people on a bus", or "the price of a packet of apples", or "a certain number."
In this case the variable is clearly
The main operation is addition - indicated by the the word "sum".
So what is being added? 18 and the variable
An expression is merely a statement, while an equation has an equal sign and not asked for on this case.
The expression is