一個實心金屬角錐體的底為4cm X 9cm的長方形，若把該實心角錐體熔掉，在鑄成一個高度相同但底為正方形的實心角金屬角錐

Let 5 cm be its height and x cm be the length of the base of the new pyramid.
5(4 * 9) = 5(x^2)
36=x^2
x=6

The length of the base of the new pyramid is 6cm.