C=34.15+0.65M
ExplanationGiven
[tex]\begin{gathered} E=15.95+0.60M \\ L=18.20+1.25\text{ M} \\ where \\ E\text{ is the cost to rental an Economy car} \\ Lis\text{ the total cost to rental an Luxury car} \\ M\text{ is the number of miles traveled} \end{gathered}[/tex]
so
to Know C, how much more it costs to rent a Luxuy can than an economc, we need to do a subtraction, hence
[tex]\begin{gathered} C(M)=L(M)-E(M) \\ \end{gathered}[/tex]
so, replace and simplify
[tex]\begin{gathered} C(M)=L(M)-E(M) \\ C=18.20+1.25M-(15.95+0.60M) \\ C=18.20+1.25M-15.95-0.60M \\ C=34.15+0.65M \end{gathered}[/tex]
therefore, the answer is
C=34.15+0.65M
I hope this helps you