Calculate and give the answer as a scientific expression.

a) [[$ 3 \cdot (\text{5.6} \cdot 10^3 -\text{6.7}\cdot 10^7)$]]

b) [[$ \dfrac{\text{6.85} \cdot 10^4 \cdot \text{9.02}\cdot 10^{12}}{4 \cdot 10^5} $]]

c) [[$ \dfrac{4 \cdot (\text{8.2} \cdot 10^9 + \text{3.1}\cdot 10^{4})}{\text{9.14} \cdot 10^{-5}} $]]