This is just a simple application of the Pythagorean Theorem, $\displaystyle a^2 + b^2 = c^2.$ In this case, $\displaystyle a=12,$ and $\displaystyle c=13.$ Solve for $\displaystyle b$. The area of the rectangle is then just $\displaystyle a\cdot b.$
Last edited by mr fantastic; Jul 21st 2009 at 01:21 AM.
Reason: Edited quote