Answer
The length of the unknown side is $\sqrt{33}$.
Work Step by Step
In the formula $a^2 + b^2 = c^2$, $c$ is the hypotenuse. Here, $a = 4$ and $c = 7$. We solve for b. Then $a^2 + b^2 = c^2$ becomes
$4^2 +b^2 =7^2$
$16+b^2=49$
$b^2 = 33$
$b = \sqrt{33}$
Cannot be simplified further