pythagorean theorum = a suqared plus b squared equals c squared. a= 3 and c = 12 so b is unknown in this case. 12 squared is 144 and then you subtract 3 squared which is 9 which is 135 so idk radical form but the answer of the equation is the square root of 135 which is approximately 11.619.
32 total rows would be required. I simply did 800, which is 4 times 200, divided by 25, which is the amount of chairs I have each row, and I got 32. Hope this helps