The third side is [tex]\sqrt{42}[/tex]
So Pythagorean's theorem is the following
[tex]Short^2 + Long^2 = Hypotenuse^2[/tex]
From here we can input the following information that we know
[tex]Short^2 + 4\sqrt{3}^2 = 8^2\\Short^2 + 12 = 64\\Short^2 = 64 - 12\\Short^2 = 42\\Short = \sqrt{42}\\[/tex]
