In one hour his father would travel 3miles(Divide both sides of 36/12 by 12)

so his son is double of that speed which would be 6miles per hour

so after 9hours, his son would travel 6milesx9hours which is equal to 54miles

so his son is double of that speed which would be 6miles per hour

so after 9hours, his son would travel 6milesx9hours which is equal to 54miles