Let:
[tex]\begin{gathered} P(x)=\text{profit} \\ k=\text{price of each t-shirt} \\ x=\text{Number of t-shirts sold} \end{gathered}[/tex]Miguel made $17.15 profit from selling 7 custom t-shirts, therefore:
[tex]\begin{gathered} P(7)=17.15=k(7) \\ 17.15=7k \\ \text{Solving for k:} \\ k=\frac{17.15}{7}=2.45 \end{gathered}[/tex]Therefore, the equation that models this relationship is:
[tex]P(x)=2.45x[/tex]