Measuring the volume of a ball that is 24cm across how can you set up an equation

The volume of every sphere is

Volume = (4/3) (pi) (radius) ³

When you say "across", I think you mean the diameter of the ball.

The radius is half of the diameter = 12 inches.

Volume = (4/3) (pi) (12 inches) ³

= (4/3) (pi) (1,728 cubic inches)

= 7,238.2 cubic inches. (rounded)