x - 5.12 = 14.88 is the correct answer.
This is because if x is the amount she had before she bought the salad, that would be her original amount. This means that $5.12 (price of salad) is subtracted from x. Since this leaves you with $14.88, you can solve if need be by subtracting $5.12 from $14.88.
Hope this helps!
