To find the cost of painting a wall, we need to find the perimeter of the wall.
Perimeter is the measure of distance of the boundary of a planar object. It is also defined as the total sum of lengths of all the bounding sides of a planar object. Whereas, area is the measurement of space enclosed by a closed planar object. Thus, to find the cost incurred to paint a wall, we need to find the area and not the perimeter of the wall.