Add or Subtract $\sqrt { 6 a } - 2 \sqrt { 54 a } + 6 \sqrt { 24 a }$

Multiple Choice

Add or subtract. Assume all variables represent positive real numbers.
- $\sqrt { 6 a } - 2 \sqrt { 54 a } + 6 \sqrt { 24 a }$

A) $7 \sqrt { 84 a }$
B) $4 \sqrt { 6 a }$
C) $4 \sqrt { 84 a }$
D) $7 \sqrt { 6 a }$