In real world, square and rectangle are different , but if you think logically both has lots of connection with it, a square can also be treated as rectangle in which both length (b) and height (h) are same (b=h), a rectangle can also be treated as square if length (b) and height (h) are equal.

You can use the Rectangle formula for Square and get the same equation, here I’m applying (b = h = a) in Rectangle formula

Perimeter of Square = 2(b + h) => 2 (a + a) = 4a
Area of Square = bh => aa = a²