we know that
Volume of the cone=(1/3)*pi*r²*h
where
r is the radius
h is the height
in this problem
diameter=x units--------> r=x/2 units
h=x units
so
Volume of the cone=(1/3)*pi*( x/2)²*x------>(1/12)*pi*x³ units³
the answer is
the volume of the cone, in cubic units is (1/12)*pi*x³ units³