I came across a problem on Khan Academy that was about finding the graph of a transformed function. The (transformed) function is given by:
$$y = 3 \log_2(2 - x)$$
What I don't understand, how can this function definition be valid for values $x > 2$? Maybe I'm missing something obvious, but isn't $\log_b x$ only defined for $b, x > 0$ and $b \neq 1$
Perhaps I have misunderstood your difficulty, but I think you have answered your own question. Indeed, as long as we are concerned with real numbers, $3\log_2(2-x)$ is not defined for $x>2$. But the graph you have posted has no points on it for $x>2$, so there is no problem.
Minor extra point: $3\log_2(2-x)$ is also undefined for $x=2$.