To answer this, first you must know the properties of a square.
With a square, each side is equal in length, which means:
4x- 50 = 60 cm
So now just solve it like a basic equation
Add 50 on both sides
4x - 50 = 60
+50 +50
4x= 110
Now divide both sides by 4
Answer is B. 27.5
