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Let's call d the distance of the cannon from us. We see the flash first, then after 6.9 s we hear the sound. The flash arrives to us traveling at speed of light, which is 
, while the sound travels at speed 
. Since the magnitude of the speed of light is much much larger than the speed of sound, we can assume that the flash arrives at our location instantaneously after the firing.
Then we can say that the sound wave covers a distance d traveling at speed v in a time of t=6.9 s. Since it is a uniformly linear motion, the distance covered by the sound wave is
So, this is the distance of the cannon from us.
Then we can say that the sound wave covers a distance d traveling at speed v in a time of t=6.9 s. Since it is a uniformly linear motion, the distance covered by the sound wave is
So, this is the distance of the cannon from us.