The given function is
[tex]f(x)=x^2-3[/tex]The expression f(a-4) means we have to evaluate the function when x = a-4.
[tex]f(a-4)=(a-4)^2-3[/tex]Now, we solve the power using the following rule.
[tex](a-b)^2=a^2-2ab+b^2[/tex]Where b = 4, and a = a.
[tex]\begin{gathered} f(a-4)=a^2-2(a)(4)+4^2-3 \\ f(a-4)=a^2-8a+16-3 \\ f(a-4)=a^2-8a+13 \end{gathered}[/tex]