the formula for the volume of a cone is
v = ( 1 / 3) h pi r^2
where h is the height of the cone
pi is 3.1416
r is the radius of the cone
since diameter is given
D = 2r
r = D/2 = 4/2 = 2 in
substitute the given
V = (1/3) (6) (3.1416) (2^2)
V = 25.1 cubic in