Proving Existence of x2=b for b≥0 in R

[kreil]

Prove that if b is greater than or equal to zero (b in R), there exists a non-negative real number x such that x²=b.

I really don't know where to go. I think I need to modify a proof done in a class that I missed, so if someone could just give me a few hints I'll fill in the rest.

Thanks
Josh

 

[AKG]

How have you defined the reals?

Or maybe prove that on [0, infinity), the function x -> x² is continuous, increasing, and unbounded, and apply intermediate value theorem.