An expression representing the following function...

So I'm stuck on this question: find an expression for the function whose graph is the bottom half of the parabola, . It should be expressed in terms of x according to the text that I'm working from.

The problem I'm having is actually isolating y without encountering a negative square root...Now, I haven't learned how to deal with complex numbers much so I'm not sure if that's the only way to do this. If it's possible to do this normally, could I have some hints?