Solving, a Square with broken diagonal: $AB=x$? I am reading a geometry lecture note and it was given this figure below.
 Where ABCD is square of size $AB=x$. $BE =12$, $EF=3$ and $FD=9$.
Please help me to find the value of $x$.
I do not know how to start. So far I know that $BD =x\sqrt 2.$ 


The hint:
Use the following.
$$\measuredangle DFB=90^{\circ}+\arctan4,$$
$$DF=\sqrt{153},$$
$$FB=9$$ and
$$DB=x\sqrt2.$$
I got $x=15$.
