Calculate with a calculator. Give the answers at the accuracy of three digits.

a) [[$ \sqrt {–9} \cdot \sqrt {–4} = $]]

b) [[$ –\sqrt {5,9} – 12 \cdot \sqrt {3} = $]]

c) [[$ \sqrt {–8 \cdot (–5)} + 7 = $]]

d) [[$ \sqrt {9 – 6 \cdot 3} = $]]