well... for an equation to be a
function, its domain must be unique, namely, the x-coordinate values in the coordinate pairs must not have any repeats.
so... first off, let's check this pairs for this expression.

notice, is just -3, -1, 1 and 3... no x-values repeat, all are different, so it IS indeed a function.
now... a distinct issue of an
inverse function is that, its domain is the range of the original function, to make it short and simple, the value pairs for the inverse of this function is simply the same value pairs flipped sideways, well, let's check the
domain of the inverse then.

well, there you have it.