Hello, john doe!

We need more information.

What is that quadrilateral on the hypotenuse . . . a square?

Is that NE-SW slanted line through the __center__ of the square?

. . And is it parallel to the sides of the square?

Assuming all that is true, we have this diagram: Code:

*
* *
A * * *
| * * *
| * * O *
| * * * *
| * /| *
| * /θ| *
| P* | *
| / Q* *
| / | *
| | *
| | *
| | 30° *
* * * * * * * * * * * * *
C R B

We have right triangle with

A square sits on hypotenuse ; is the center of the square.

. . Actually, it could be __any__ point in the square!

In right triangle

Then .(vertical angles)

Therefore, in right triangle