What is a special set of ordered pairs where the domain cannot repeat

I’m stuck on this equation, anyone got the answer?

Rose bought a new hat when she was on vacation. the following formula describes the percent sales tax rose paid b=t-c/c

The spanish club held a car wash to make money. the equation y=5x represents the amount of money y club members made for washing x cars. identify the constant of proportionality. then explain what it represents in this situation