Sure, let me help you on this one!
If Larissa earns $500 per week, she will need $250 more in order for her to reach the desired salary of $750 per week.
I am afraid you are lacking some information in the question, since you didn't specify the percentage of the commission Larissa is permitted to take, but for the sake of helping you understand the structure of this question, I will make it 100% - all of the sales money go to Larissa.
If that's true, Larissa will only need to sell enough stuff for $250.
If the commission was 50%, Larissa would have to sell enough stuff for $500 since 50% of her sales are taken.
I hope this helped you, and feel free to re-ask me this question when you have all of the information required.