its a semicircle, so just follow the formula for area of a circle then divide the answer by 2
In other words:
A=(3.14)r^2
A=3.14*20^2 (because the diameter is 40, and diameter is twice the radius)
A=3.14*400
A=1256
I think i got a correct answer, but let me know if i was wrong thanks!

Baileysheen

you just need to do the formula which is A=PI TIMES RADIUS SQUARED