I assume that after writing an expression it needs to be solved? Let's begin by breaking the question into parts...

sofia owes her brother $25..this represents a total so keep that in mind...

she gives her brother the $18 she earned dog-sitting...

So from Sofia's perspective the $25 is a bill. She paid the brother $18 thus "deducting" $18 from the $25 bill. See where this is going? So your expression would be 25-18. This is a subtraction expression however.

We do know that she paid $18 off the bill for $25 so we could subtract 25 from 18 which is...7.

A valid addition expression would be x + 18. This reads "18 more than a value x". If you want to replace x then do it like this...."7 + 18". This would be your addition expression and the answer you are looking for. Do you need to calculate the sum as well? Well 7 + 18 is what? 25. Which is exactly how many total dollars Sofia owes her brother!