The formula for the area of a circle is A = pi x (radius^2).
We need the radius of the circle in order to use the formula. The radius is equal to 1/2 of the diameter of the circle. The radius goes from the center to one side of the circle, whereas the diameter touches two sides of the circle and goes through the center.
radius = 6 / 2 = 3 units
Remember that a semi-circle is equal to 1/2 of a full circle. So we need to find the area of the whole circle and divide it by 2.
A = pi x 3^2
A = 9pi
A = 4.5pi = 14.13 units^2
Hope this helps! :)
