#### Answer

3$\sqrt 11$

#### Work Step by Step

The Pythagorean Theorem says that $(leg_{1}) ^{2}$+$(leg_{2})^{2}$=$hypotenuse^{2}$.
This formula is more commonly referred to as $a^{2}$+$b^{2}$=$c^{2}$
You are given the lengths of one leg as 15 and the length of the hypotenuse as 18. Substitute 15 in for a and 18 in for c.
$a^{2}$+$b^{2}$=$c^{2}$
$15^{2}$+$b^{2}$=$18^{2}$
225+$b^{2}$=324
Subtract 225 from both sides
$b^{2}$=99
Take the square root of both sides to find the length of the missing side
b=3$\sqrt 11$