#### Answer

$a=2\sqrt 14$

#### Work Step by Step

According to the Pythagorean Theorem, if $a$ and $b$ are the lengths of the shorter sides of a right triangle and $c$ is the length of the longest side, then $a^{2}+b^{2}=c^{2}$.
Therefore, $a^{2}+5^{2}=9^{2}$.
$a^{2}+25=81$
Subtract 25 from both sides.
$a^{2}=56$
Take the square root of both sides.
$a=\sqrt 56=\sqrt (4\times14)=\sqrt 4\times\sqrt14=2\sqrt 14$