How do I evaluate a radical function?

Question

How do you solve this square root function? $f(x+9)=\sqrt{x+9}$. I tried to solve it but I don't know what I should do after $\sqrt{x+18}$.

Answer 1

Substitute $x = y-9$.

Then, $f((y-9)+9)=\sqrt{(y-9)+9}$, so $f(y)=\sqrt{y}$.