12．某离散型随机变量$X$的分布列如下，若$E(X)=\frac{3}{4}$，$P(X\geqslant 1)=\frac{7}{12}$，则$D(X)=($　　$)$
------------------------- ------------------------- ------------------------- ------------------------- ---------------------------
$X$ $-1$ 0 1 2
$P$ $a$ $b$ $c$ $\frac{1}{3}$
------------------------- ------------------------- ------------------------- ------------------------- ---------------------------