How to cal. Area of a circle in C# pls...I've tried it for hours now, and i still don't get it

Answer 1

area = radius * radius * Math.PI

Answer 2

Another one to the former would be:
int radius;
double area = Math.Pow(radius, 2)*Math.PI;
Don't forget to make that with the Datatype double or when you want it to be int use:
-Math.Ceiling() //Round next highest integer
-Math.Floor() //Round to lowest
-Math.Round // Round low from 0-5 and hight from 6-9