Answer
$c^{}= 1050.95$ ft/s
$V= 5020$ mi/h
Work Step by Step
The temperature is $$0+459.67=459.67 \mathrm{R}.$$ The speed of sound is $$
c=\sqrt{k R T}=\sqrt{(1.4)(0.06855 \mathrm{Btu} / \mathrm{lbm} \cdot \mathrm{R})(459.67 \mathrm{R})\left(\frac{25,037 \mathrm{ft}^2 / \mathrm{s}^2}{1 \mathrm{Btu} / \mathrm{lbm}}\right)}=1050.95 \mathrm{ft} / \mathrm{s}
$$ and $$
V=c \mathrm{Ma}=(1050.95 \mathrm{ft} / \mathrm{s})(7)\left(\frac{1 \mathrm{mi} / \mathrm{h}}{1.46667 \mathrm{ft} / \mathrm{s}}\right)=5015.9 \mathrm{mi} / \mathrm{h} \cong \mathbf{5 0 2 0} \mathbf{~ m i} / \mathbf{h}
$$
