By inspecting the diagram, we find that the distance from X to Y, given as 42 units, is also the diameter of the circle. The typically-used formula for the circumference of a circle is 2*pi*r. Since 2r is the same as the diameter, we can substitute that for the 2 and the r in the equation: Circumference = pi*diameter Circumference = 3.14*42 Circumference = 131.88