Simplify Write the Answer Using Positive Exponents Only $( - 3 a ) ^ { 2 }$

Simplify. Write the answer using positive exponents only.
- $( - 3 a ) ^ { 2 }$

A) $9 a ^ { 2 }$
B) $- 6 a ^ { 2 }$
C) 9a
D) -6 a