The limit exists at all values of x₀ where the function is continuous, i.e. where there is no break in the graph.
So only consider the points where there is a vertical asymptote (x₀=-6), where there are holes and jump discontinuity (x₀=-3,3).
The vertical asymptote is a place where the function is undefined and the limit of the function does not exist.
Hence, the limit does not exist at x₀=-6.
For the point where there is a hole, x₀=-3, notice that the graph approaches the same y-value both from the left and right, hence the limit exists at this point, as this is a removable discontinuity.
For the point, x₀=3 where there is a jump discontinuity, notice that the graph approaches different values from the left and right, respectively. Hence, the left and right limits are not equal and thus the limit does not exist.
So the limit exists over the set of real numbers except {-6,3}.