I have to find the Fourier transform of $[3^{(-k)}]u(-k-1)$
so the Fourier transform $X(f)= x(k) e^{(-j\cdot 2\pi f k)}$.
I have $(1/3)^{k} u[-k-1]$
The problem is, I know that the Fourier transform of $(1/3)^{k} u[k-1]$ is $$[(1/3) e^{-j\cdot 2\pi f}]\over[1-(1/3) e^{-j\cdot 2\pi f}]$$
but not when we have $-k$ instead of $k$?