Can somebody explain how to find the answer? I'm confused.

Answer: [tex]f(4a)=\frac{4a+6}{16a^2+1}[/tex]
Step-by-step explanation:
f(4a) means everything you see x, plug that in with 4a. Doing that exactly so, we can see that below.
[tex]f(x)=\frac{x+6}{x^2+1}[/tex] [plug in x=4a]
[tex]f(4a)=\frac{4a+6}{(4a)^2+1}[/tex] [exponent]
[tex]f(4a)=\frac{4a+6}{16a^2+1}[/tex]
Since this cannot be simplified, [tex]f(4a)=\frac{4a+6}{16a^2+1}[/tex] is our answer.