Consider this option:
1. area_rombus=a*h, where a=6 - the length of the side, h - height.
h=area_rombus/a.
2. area_sq=a², where a=6 - the length of the square.
area_sq=36, area_rombus=4/5 *36=28.8.
3. according to the item 1 h=area_rombus/a=28.8/6=4.8.
answer: 4.8
