[tex]\text{Apply Pythagorean theorem,}~ \text{Hypotenuse side}^2 = \text{Opposite}^2 + \text{Adjacent side}^2[/tex]
[tex]12^2 =y^2 +8^2\\\\\implies y^2 = 12^2 -8^2 \\\\\implies y^2 = 80\\\\\implies y =\sqrt{80} = \sqrt{16\times 5}= 4\sqrt 5~~ \text{Units}[/tex]
