Question

Let
f(x) = 2x/(x^2+9).
Find the domain of f. Write your answer using interval notation.

Answer 1

since the denominator can never equal to zero and it contains no square roots or other strange operators,

domain : x ∊ R

I like the "old fashioned" format I used, I always have to look up this "interval notation stuff"
I will let you write it your way.