Olivia spent 6 dollars at a supermarket. If she has 8 dollars left with her How much money did she have initially?
Olivia spends 6 dollars at the supermarket. 
That means she had 6 dollars more than she does now. 
That means she has 8 - 6 = 2 more dollars than she started with. 
That means she started with 8 - 2 = 6 dollars.