#### Answer

$a=\sqrt{b^{2}+c^{2}}$

#### Work Step by Step

The length of AB can be calculated: $x_{B}-x{A}=a$
The length of AD can be calculated with the Pythagorean Theorem:
$\sqrt{(x_{d}-x_{a})^{2}+(y_{d}-y_{a})^{2}} = \sqrt{(b-0)^{2}+(c-0)^{2}}$
So, $a=\sqrt{b^{2}+c^{2}}$