Remember that the volume of a cone is equal to 1/3 base area times height. If the height of the taller cone is a factor x times larger than the smaller cone, then the radius of the base of the larger cone is that same factor x times larger as well, because the two cones are similar. This means the area of the base of the larger cone is times larger than the area of the base of the smaller cone - do you see whythat is? Consequently the volume of the larger cone is a factor times bigger than the volume of the smaller cone. From this - can you determine what the value of x must be be for the larger cone to be twice the volume of the smaller cone?