#### Answer

$y=\sqrt{433}$

#### Work Step by Step

The Pythagorean Theorem states that for any right triangle, $a^{2} + b^{2} =c^{2},$ where c is the length of the hypotenuse and a and b are the length of the other two sides. Plugging in the dimensions of the given triangle into the Pythagorean Theorem, we obtain:
$$ 12^2 +17^{2}=y^{2} \\ y^2 = 433 \\ y=\sqrt{433}$$