a) $(2-i) \cdot (3+2i) = $ $2 \cdot 3 + 2 \cdot 2i -3i -2i^2 =$ $6 + 4i -3i -2 \cdot (-1) =$ $8 + i $
$8 + i \in C $ b) $\left( \frac27+\frac314\right): \frac15 = $ $ \frac714: \frac15 = \frac3514 = \frac52$
$\frac52 \in Q , R$ c) $7.3 \cdot 10^-4 + 1.2 \cdot 10^-2 = $ $0.073 \cdot 10^-2 + (…)