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

a) [[$ \sqrt {15 – 9} = $]]

b) [[$ \sqrt {2 \cdot 4 + 6} = $]]

c) [[$ \sqrt {2^2 + 3^2} = $]]

d) [[$ \sqrt {6 \cdot 7 – 4^2} = $]]