It would be D. If you add all the numbers (-18, -81, 144) and divide it by 3 then you would get -81. I hope I helped!
Step-by-step explanation:
We have y = 13 - 3x and y = 7 - x.
=> 13 - 3x = 7 - x
=> 2x = 6, x = 3.
Then y = 7 - (3), y = 4.
Hence the price of a pencil is $3 and the price of a notebook is $4. (C)
