My assumptions
a) The house is rectangular.
b) You want to find the dimensions as area is already given else the area is 561 cm^2
c) It is a toy house, a real house cannot be this small.
Solution:
Let length of the house be l and breadth of the house by b.
Given,
l/b = 3/8
or l = 3b/8 ----------------(1)
Also, area of the house is given by length * breadth = l * b = 561
=> 3b/8 * b from equation (1)
=> (3b^2)/8 = 561
=> b^2 = 561*8/3
=> b = sqrt(1496) = 38.6781592116
So b = 38.6781592116 cm
and l = 3b/8 = 14.5043097043 cm