Look at the square on the left.

Its bottom right corner angle = ∠ a + ∠b + ∠ c

Now look at the middle square.

Its bottom corner angle = ∠ b + ∠ c + ∠ d

Since the corners of a square have angle 90° (right angle), both these corner angles equal 90°

∠ a + ∠ b + ∠ c = 90°
∠ b + ∠ c+ ∠ d = 90°

Notice that both angles have ∠ b and ∠ c in common.

Since the sum equals 90° for both ,
∠ a = ∠ d in order to give the same sum

It is the same reasoning for why ∠ b = ∠ e
(Compare the corners of the middle and right square)