The area of an equilateral triangle whose vertices are $(-a,-a),(a,a)$ and $(-\sqrt {3a}, \sqrt {3a})$ is ________

Question

$\begin{array}{1 1}(A) \;6\;a^2\;sq.units\\(B)\;3 \sqrt 2 \;a^2\;sq.units \\(C)\;2\sqrt 3 \;a^2\;sq.units \\(D)\;12\;a^2\;sq.units \end{array}$