y = ( 3 * x ) - 2.
This is because the card costs for 3 $ and should be multiplied by the number of total cards she buys in that particular day. So 3 * x.
Now as she has got a 2$ off coupon, whatever the total is she gets 2 dollars off on that price. So we will subtract 2 at that time. FInally all this summed up would be equal to y.