Let L be length and B the breadth. Perimeter = 2*(L+B) = 84 L+B = 42 If 4 cm is taken from length it becomes L - 4 If 4 cm is added to breadth it becomes B + 4 Since it becomes a square, and all sides of square are equal, we have L - 4 = B + 4 L = B + 8 Since L + B = 42, Substituting for L as B + 8 B + 8 + B = 42 2*B = 34 B = 17 cm Length = B + 8 = 25 cm Hope you got it :)