A girl scout has (5x-12) boxes of cookies and sells (3x+18) of them. Write an expression to represent the amount of boxes she has left. Also can you show me the work that gets this answer not just the answer

You have to subtract 3x + 18 from 5x - 12:

=> 5x - 12 - (3x + 18) = 5x - 12 - 3x - 18 = 5x - 3x - 12 - 18 = 2x - 30

The first step was to apply distributive property to supress the parenthesis, after that the terms were arranged and the like terms combined.

Answer: 2x - 30