How I can use the real root of a symbolic subs expression?

I am using the symbolic function

$$f = \sqrt[3]{x^2-9}$$

but the command subs return a complex result for $x$ negative. How I can return the real root?

thanks!

Answer 1

[Walter Roberson]

http://www.mathworks.com/help/symbolic/mupad_ref/numeric-realroots.html