You can put this solution on YOUR website! Inverse for y=-3 is not a function, but would still be a relation. y=-3 will include ALL values of x but just ONE value for y. When you switch roles of x and y, you would have x=-3 for ALL values of y. This is x=-3.
f^-1 from your definition of f(x) gives . .
Graphically like these:
f is the red line, and its inverse is the green line.