Simplify the Expression $( \sqrt { 6 } - 7 ) ( \sqrt { 7 } + 3 )$

Multiple Choice

Simplify the expression. Assume all variables represent positive real numbers.
- $( \sqrt { 6 } - 7 ) ( \sqrt { 7 } + 3 )$

A) $\sqrt { 42 } - 4 \sqrt { 7 } - 21$
B) $- 3 \sqrt { 42 } - 21$
C) $\sqrt { 42 } - 21$
D) $\sqrt { 42 } + 3 \sqrt { 6 } - 7 \sqrt { 7 } - 21$