If you rolled a sheet of paper of length x and width h to form a cylinder then the height of the cylinder is h and the circumference of the end of the cylinder is x.
If r is the radius of the circle at the end of the cylinder then the circumference is 2 π r. But the circumference is x so
x = 2 π r
The area of the sheet of paper A is x times h and hence
A = 2 π r h
Solve this equation for r and substitute the value for r into the expression you know for the volume of a cylinder v = π r2 h.