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#