How to remove non negativity constraint(i.e. make it a free variable)?

Question

I am currently trying to show a polyhedron $P = \{Ax=b, x\ge0\}$ can be written in the form $P = \{Ax\le b, x \in R^n\}$. I know $Ax=b$ can be written as $Ax\le b$ and $-Ax\le -b$, but I am not sure how to deal with the non-negative constraint on $x$.