Solve the Quadratic Equation by Completing the Square and Applying $$3 x ^ { 2 } - 12 x + 21 = 0$$

Multiple Choice

Solve the quadratic equation by completing the square and applying the square root property. $$3 x ^ { 2 } - 12 x + 21 = 0$$

A) $\{ 2 + \sqrt { 3 } , 2 - \sqrt { 3 } \}$
B) $\{ 2 + i \sqrt { 3 } , 2 - i \sqrt { 3 } \}$
C) $\left\{ \frac { 3 } { 2 } , - \frac { 3 } { 2 } \right\}$
D) $\{ - 2 + i \sqrt { 5 } , - 2 - i \sqrt { 5 } \}$