Prove proposition on real numbers and uniqueness.

How would I go about proving the following proposition. Do I have to prove uniqueness, or that if $x^2 = r$, then $x = \sqrt r$?

Prove given any $r \in \mathbb R\gt 0$, the number $\sqrt r$ is unique in the sense that, if $x$ is a positive real number such that $x^2 = r$, then $x = \sqrt r$.