Answer:
y=3*x
Explanation:
john bought 3 notebooks for 9 dollars
so 9/3=3
notebooks costed 3 dollars each
x-number of notebooks
y-cost of notebooks
so
y=3*x
because 3 dollar cost every notebook you could buy.
if i buy 2 notebooks
so x=2
y=3*2=6 dollars
if I buy 3 notebooks
then y=3*3=9 dollars
end this way continues