Find the Domain of the Given Function $$f ( x ) = \sqrt { 21 x - 7 }$$

Find the domain of the given function.
- $$f ( x ) = \sqrt { 21 x - 7 }$$

A) all real numbers $x$ where $x >$
B) all real numbers $x$ where $x > - \frac { 1 } { 7 }$
C) all real numbers $x$ where $x \geq - 7$
D) all real numbers $x$ where $x \geq -$