Hey there! I'm happy to help!
The additive inverse of a number is one that has its sign switched. If you put a - before a number, the sign switches. We put a - before -5, so its sign switches. -5's additive inverse is 5, so this 18+5, giving us 23.
The additive inverse of -5 is +5, so 18 - (-5)=23.
Have a wonderful day! :D