Longanecker396
38

Use the Pythagorean theorem $a^{2} + b^{2} = c^{2}$ to find the missing side length of a right triangle  In this case, after you plug in the given numbers your equation is $12^{2} + y^{2} = 20^{2}$ Then, Just solve that equation for y. $144 + y^{2} = 400$ $x^{2} = 256$ $\sqrt{} y^{2} = \sqrt{256}$ y = 16 So the answer is B, 16 cm. I hope this helps, please feel free to ask questions if you're still confused :)