There were w dollars in Olivia's wallet. She collected x more dollars from an atm. After she visited a supermarket there were x dollars left. How much more money did she collect at the atm than she spent at the supermarket?
Olivia had w dollars in her wallet. 

She collected x more dollars from an ATM. 

That means she had w + x dollars in her wallet. 

After she visited the supermarket, she had x dollars left. 

That means she spent w + x - x = w dollars at the supermarket. 

So she collected x - w more dollars from the ATM than she spent at the supermarket.