1) x² + y² = 64 is the EQUAION of a cercle with a radius R = √64 =8
2) The cross section is a triangle with H = BASE.
Since R = 8 that means the Base B= 16 and also the height H =16
This solid is then a regular cone and its Volume value is
V = πR².H
V π(8)².(16) = 2014π units³ or ≈ 3217 units³