If I understand your question correctly, you have given me the altitude (16 cm) between the two 81 cm sides. You would like to find the altitude between the two 24 cm sides. Right?

The area of a parallelogram is A=bh.

Using 81 cm as a base and 16 cm as an altitude, we have A=(81)(16)=1296 square cm.

If we find the area using the other base of 24, we have A=24h.

Since the areas are the same, we have 24h=1296.

Thus, h=54 cm.