Area of a circle = pi x radius squared
Radius of a circle is from any point on the circumference to the centre
Diameter is the distance between two points on the circumference with the line crossing the centre
So radius is half of the diameter...
In this example radius is therefore 9mm (half of 18mm)
Square this... 9x9=81
So in terms of pi the answer is 81pi
To get the full answer you would just times pi (3.14) by 81 =254.34