Answer
$Ma= 0.974$
Work Step by Step
The temperature is $$-50+273.15=223.15 \mathrm{~K}.$$ The speed of sound is
$$
c=\sqrt{k R T}=\sqrt{(1.4)(0.287 \mathrm{~kJ} / \mathrm{kg} \cdot \mathrm{K})(223.15 \mathrm{~K})\left(\frac{1000 \mathrm{~m}^2 / \mathrm{s}^2}{1 \mathrm{~kJ} / \mathrm{kg}}\right)}\left(\frac{3.6 \mathrm{~km} / \mathrm{h}}{1 \mathrm{~m} / \mathrm{s}}\right)=1077.97 \mathrm{~km} / \mathrm{h}
$$ and $$
\mathrm{Ma}=\frac{V}{c}=\frac{1050 \mathrm{~km} / \mathrm{h}}{1077.97 \mathrm{~km} / \mathrm{h}}=0.9741 \mathrm{~km/h} \cong 0.974 \mathrm{~km/h}
$$
