Area of the square

Can you find the area of the following square given the known lengths?

Good luck!

• just checking: did you make this yourself? – Omega Krypton Sep 17 '19 at 1:50
• No. I've been sent this problem by a friend, but I do not have the original source. – Dmitry Kamenetsky Sep 17 '19 at 1:51
• This is a classic puzzle. – greenturtle3141 Sep 17 '19 at 3:52

$$\sqrt{(9+3)^2+5^2} = \sqrt{169} = 13$$
$$(\frac{13}{\sqrt{2}})^2 = \frac{169}{2}$$