The function ƒ (x) = x2 + a is not one-to-one. Find a portion of the domain where the function is one-to-one and find an inverse function.

Once you know what one-to-one means it gets a lot easier.

A function is one-to-one if for each x value there is one and only one f (x) value.

For your function lets say x=1 or x=-1, if this is a one-to-one function then the f (x) values will have to be different. Check if this is true here