  In this paper, we introduce a ring isomorphism between the Clifford algebra
$C\ell_{1,2}$ and a ring of matrices. By such a ring isomorphism, we introduce
the concept of the Moore-Penrose inverse in Clifford algebra $C\ell_{1,2}$.
Using the Moore-Penrose inverse, we solve the linear equation $axb=d$ in
$C\ell_{1,2}$. We also obtain necessary and sufficient conditions for two
numbers in $C\ell_{1,2}$ to be similar.
