I think you know this but just in case you dont the radius is half of the diameter. That's why I put 5 instead of 10. :)
Each brother should get 4 slices. 12 slices divided by 3 brothers equals 4 slices each.
√(0+5)^2+(9+3)^2. …two negatives make a positive so that’s how both 3 and 5 got flipped to positive.
√ 5^2+(9+3)^2. Remove the 0 since 5+0=5
√25+12^2. Evaluate
√ 25+134. Then just add
√ 169. Evaluate
13 is the answer