Let
$$
\mathcal{C}:=\left\{\left(\begin{array}{rr}
a & -b \\
b & a
\end{array}\right) ; \quad a, b \in \mathbb{R}\right\} \subset M(2 \times 2 ; \mathbb{R})
$$
with ordinary addition and multiplication of (real) $2 \times 2$ matrices.
Show: $\mathcal{C}$ is a field, which is isomorphic to $\mathcal{C}$, the field of complex numbers.