This is a rhombus. A property of a rhombus is that their diagonals perpendicularly bisect each other. In other words, if the point in the center is A, then HA = MA and EA = OA. So: 3 = a and 4 = b.
Since angle HAO forms a right angle, we can use the Pythagorean Theorem to say that HA^2 + AO^2 = OH^2: 3^2 + 4^2 = 9 + 16 = 25.
