Use the Pythagorean theorem. For a right triangle with legs $a$ and $b$ and hypotenuse $c$:

$c^2 = a^2 + b^2$

Substitute $a = 8$ and $b = 3$:

$c^2 = 8^2 + 3^2 = 64 + 9 = 73$

$c = \sqrt{73}$