Measuring is a skill that is often learned in grade school. During these years, we learn in mathclass about the area of shapes. There are different formulas for measuring the area of different ways. For example, a triangle has a different method to find the area of it than a square. A triangle’s area is found by multiplying half the height times the base.
A square’s area is measured by multiplying the side by the other side. If there is a shed and you know the dimensions of the walls, then you can calculate the area of the wall based on its shape. You would need to see the formula though for that shape. In this case, the city would be 30.