To overestimate the area would mean that you completely enclose the circle in squares; an 8 x 8 square to b exact. The area of a square with sides measuring 8 is 64. The overestimation is 64.
To underestimate the area would mean that you only count the squares that are completely inside the circle. That would be a 6 x 6 square (even though the 6th squares are a tiny bit outside the circle). A square with sides of 6 has an area of 36. The underestimation is 36.
