split it up
small square on top with side legnths s and s
big squaer with legnth and width 5s and 3s
area=legnth times width
total area=5s*3s+s*s=15s^2+s^2=16s^2
toatl area=600
600=16s^2
divide both sides by 16
37.5=s^2
squer root both sides6.12372
tenth
6.1 mm is s
s=6.1mm