The function f(x) = 4 is simply the horizontal line y = 4. Therefore, the region bounded by the curve of the function f(x) = 4 on the interval [1, a] with a > 1 is simply a rectangle whose length is (a-1) units and width 4 units. We have been informed that the area is 20 square units, therefore we can formulate the following equation using the formula for the area of a rectangle;
