The graph shows the function f(x)=3^x what is the value of x when f(x)=9? a: 3 b: 1 c: 0 d: 2

The answer is D.
You have to make the whole numbers equal to then solve for the exponents:
F(x) means y, therefore, y is 9.
9 = 3^x.
3^2 is the same as 9.
Then, 3^2 = 3^x.
Then solve for the exponents,
2 = x
Therefore your answer is 2.