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