Hello, when talking about the capacity of something they are always talking about the volume of an object. In this case the object is a tank and I am assuming based on the information that it is a cylindrical tank, or a tank in the shape of a cylinder. The volume of a cylinder is based on the area of the base (which is a circle) and then you take that area and you multiply it by the height of the cylinder, in this case it is 110 feet, in order to get the volume. So as long as you can find the area of the circle just take that value and multiply it by the height and you should have your answer. One quick pointer,
area of a circle = π (radius)2 and in this question you are given the diameter which is 2 x (radius).
Hope this helps,