Multiply, and Then Simplify If Possible $( 7 \sqrt { x } + 3 ) ( \sqrt { 7 x } - 3 )$

Multiple Choice

Multiply, and then simplify if possible. Assume all variables represent positive real numbers.
- $( 7 \sqrt { x } + 3 ) ( \sqrt { 7 x } - 3 )$

A) $7 x \sqrt { 7 } - 42 \sqrt { x } - 9$
B) $7 x \sqrt { 7 } - 21 \sqrt { x } + 3 \sqrt { 7 x } - 9$
C) $7 x - 9$
D) $7 x - 42 \sqrt { x } - 9$