calculus - How to find the infinite sum

I need to find the infinite sum of the following series expansion

$$1/3 + 2/3^2 + 3/3^3 + 4/3^4 + \dots + k/3^k + \dots$$

I know that

$$x/(1 - x) = x + x^2 + x^3 + \dots + x^k + \dots$$

We need to find the $x$ value in order to find the infinite sum. What could the $x$ value be? I am not sure.