The answer is 3, because the other two sides are 3 and the angle is 60, so the other angles will have to be 60. That means it is an equilateral triangle.
Since it was said that the diameter at the base is 36 feet and an person with an outstretched arm would be 6 feet then 6 times 6 is 36 or you can do the division part which will be 36 divided by 6 and you will still get the same answer: 6