Answer
The speed of sound at the airplane's altitude is $~~1170~km/h$
Work Step by Step
We can find the speed of sound:
$sin~\theta = \frac{v}{v_S}$
$v = v_S~sin~\theta$
$v = (1350~km/h)~sin~(60^{\circ})$
$v = 1170~km/h$
The speed of sound at the airplane's altitude is $~~1170~km/h$
