I'm really sorry that I took a long time. I got carried away. Hopefully my work was correct though.
A= about 354 square feet~ (rounded)
The stage can be split into two shapes. The first is a trapezoid and the formula to find the area for it would be A=(b₁+b₂)/2 and then multiple by the height. b₁ would be 17 and b₂ is 20 and the height is 13. 17+20 is 37. 37/2 is 18.5. 18.5 times 13 is 240.5. Area of the trapezoid is 240.5 square feet. Then we need to find the area of the semicircle. The formula is A=(πr²)2. r is the radius which is half of the diameter. The diameter is 17 so the radius is 8.5. π is about 3.14 if you don't have a calculator. 8.5 squared is 72.25. 72.25 times 3.14 is 226.865. 226.865/2 is 113.4325. The area of the half circle is 113.4325. Just add these two areas to get 353.9325. So I think the area is pretty much 353.9325 but might be slightly different because some values I said were rounded. Either way the answer should be similar.
I forgot you said you needed help fast and got carried away. Sorry if I wasn't fast enough and hopefully I did my math correctly.