If x is the length of the unknow side, then, you have that the area of the square is:
A1 = x^2
Now, take into account that the area of the previous square is five times the area of other square with side 6.
The area of the other square is then:
A2 = 6^2 = 36
and A1 is five times A2, that is:
A1 = 5*A2 = 5*36 = 180
Next, consider that A1 is equal to x^2:
A1 = 180 = x^2
By applying square root both sides you obtain for x:
x = √180 ≈ 13.42
The length of the side is approximately 13.42 units