#### Answer

$x = \sqrt {170}$

#### Work Step by Step

We can find the third side by using the Pythagorean theorem, which states that $a^2 + b^2 = c^2$, where $a$ and $b$ are the legs of the right triangle and $c$ is the hypotenuse.
Let's plug in what we know into the Pythagorean theorem:
$7^2 + 11^2 = x^2$
Evaluate the exponents:
$49 + 121 = x^2$
Add to simplify:
$170 = x^2$
Take the positive square root to solve for $x$:
$x = \sqrt {170}$