A real-world description of 5x - 18 could be the calculation of net income for a company in a particular month.
For example, if x represents the revenue generated by the company in that month, then 5x could represent the total amount of sales revenue, and 18 could represent the total expenses incurred by the company in that month.
Subtracting the expenses from the revenue would give you the net income for the month, which would be represented by the expression 5x - 18.