Solution (source code)

= Solution

Choose the <linear relation>
$$
\boxed{c=Md,\qquad c_i=M_{ij}d_j.}
$$
Then the displayed <tensor contraction> is $(a_ic_i)b_k-(a_ib_i)c_k=(a\cdot c)b_k-(a\cdot b)c_k$, precisely the $k$th <vector component> of $a\times(b\times c)$ by the <vector triple product> identity. The <matrix> $M$ need not be a <symmetric matrix>.