Answer
$c^{}= 318.93$ m/s
$V= 8040$ km/h
Work Step by Step
The temperature is $$-20+273.15=253.15 \mathrm{~K}.$$ The speed of sound is $$
c=\sqrt{k R T}=\sqrt{(1.4)(0.287 \mathrm{~kJ} / \mathrm{kg} \cdot \mathrm{K})(253.15 \mathrm{~K})\left(\frac{1000 \mathrm{~m}^2 / \mathrm{s}^2}{1 \mathrm{~kJ} / \mathrm{kg}}\right)}=318.93 \mathrm{~m} / \mathrm{s}
$$ and $$
V=c \mathrm{Ma}=(318.93 \mathrm{~m} / \mathrm{s})(7)\left(\frac{3.6 \mathrm{~km} / \mathrm{h}}{1 \mathrm{~m} / \mathrm{s}}\right)=8037 \mathrm{~km} / \mathrm{h} \cong 8040 \mathrm{~km} / \mathrm{h}
$$
