7x + 2y = 27
I can't read what the variables are in the picture, so I used x and y. But replace the variable 'x' with whatever the question says to use for notebooks, and change y to be whatever it says for the second thing in the store.
What my equation means is that an unknown number of notebooks, which is 7$ each, plus the other thing, which is $2 each, comes to a sum of $27.