Cho hình chóp $S.ABC$ có đáy $ABC$ là tam giác đều cạnh $2a$. Gọi $M$ là trung điểm cạnh $SA$,$\widehat{SAB}=\widehat{SCB}=90{}^\circ $, biết khoảng cách từ $A$ đến $\left( MBC \right)$ bằng $\frac{6a}{\sqrt{21}}$. Thể tích của khối chóp $S.ABC$ bằng

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