You want to start by finding x. x in this equation is the amount of money that it will cost to rent a tent for one day. You are told that it costs $56 to rent a tent for 7 days.
If you divide both numbers by 7, you will see that it costs $8 to rent a tent for 1 day. You multiply the cost by the number of days you rent it. So 2 days would be $16, 3 days would be $24 ... etc.
Therefore, the equation is
y("cost")= 8("dollars") times x("days").
y=8x