In the given function the vertex (h,k) is (-1,0).
To find the inverse of the given function as it is not a one to one function (has the same value f(x) for more that one value of x) you restrict the domain to get a function one to one.
In a quadratic function as given use the vertex to restric the domain; restric the domain using coordinate x of the vertex.
In the given equation the coordinate x of the vertex is -1, then you can restirc the domain as follow: