Answer
$4$
Work Step by Step
The Pythagorean Theorem says that for a right triangle (if $z$ is the hypotenuse and $x,y$ are the other sides): $x^2+y^2=z^2$.
Hence here if we denote the unknown length with $x$:
$x^2+3^2=5^2\\
x^2+9=25\\
x^2=25-9\\
x^2=16\\
x=\pm\sqrt{16}\\
x=\pm4$
Since a side length is positive, then $x=4$.
