a — height of a white tile

b — width of a white tile

a x b — S₁, area of a white tile (this is what we are trying to "find")

(a - b)² — S₀, area of a black square

(a + b)² — S, area of a whole block (4 white tiles, 1 black tile)

Now, looking at the image it is obvious, that (S - S₀) = 4S₁.