We are given two functions: One that represents the salary of Eric for selling x fish tanks and one that represents the salary of Nancy for selling x fish tanks.
Eric's formula would be
and Nancy's formula would be
we are told that we want to calculate their earnings if they both sell 4 fish tanks each. So, we take this formulas, replace the value of x by 4 and then check how makes more.
For Eric, his earnings would be
and for Nancy, her earnings would be
So, in this case, Eric would earn more than Nancy for selling 4 fish tanks.