Quadrilaterals and Pythagoras Theorem

Hi, new to this,

The diagonals AC and BD, of a quadrilateral ABCD, are perpendicular. Find the length of AB, if BC=5cm, DC=4cm and AD=3cm. Let the intersection of the diagonals be E.

Now I'm thinking the answer will be in surd form. I've been thinking about the question for a while but i don't really know how to go about answering it.

Thanks in advance.:)

Thanks, want to clear something up.

Thanks for the help,

So is

$\displaystyle (f^2 + g^2) - (g^2 + h^2) + (h^2 + j^2) = f^2 + j^2 = x^2$

because that essentially is the same as

$\displaystyle f^2 + g^2 + h^2 + j^2 - g^2 - h^2 = f^2 + j^2 = x^2$

The two $\displaystyle g^2$'s and $\displaystyle h^2$'s cancel out, equalling $\displaystyle x^2$

Thanks:)