For this case we can propose an equation that will allow us to find the price of one of the books knowing the price of the other.
x: Let the variable that represents the price of the book with a green sticker
y: Let the variable that represents the price of the book with a yellow sticker
According to the statement we have:
Finally, we can substitute the price of one of the books and clear the value of the other as the case may be.
Answer: