The fossil in the picture is a trilobite that lived about 500 mya. so, the age of the rock in the picture is million years old. a) at least 500 b) at most 500 c) exactly 500

The correct answer is A) at least 500.

The rocks in which the fossils are found, are more often then not, indicators of the age of the fossil itself. So if the estimate of the fossil's age is 500 MYA then the rock is of at least the same age, most of the time older then the fossil. 

I'm pretty sure there's a geometry rule for   anyways, if the triangle is equilateral, and the circle is circumscribed, the circle is outside of the triangle.  to find the area, you need the radius.  the radius should be a straight line from the center of the triangle to any of it's points.  draw lines from the center of the circle to each of the triangle points.  draw three more lines from the center to the middle of the sides of the triangle.  you should now have six separate 30-60-90 triangles inside of your original triangle. use your 30-60-90 rules to solve for the radius, then the circumference.