$$\\c^2=161.44\\\\
c=\sqrt{161.44}\\\\$$
Just do it straight on the calc on the home page.
You can either type it in as sqrt(161.44) or you can use the $$\sqrt$$ sign
$${\sqrt{{\mathtt{161.44}}}} = {\mathtt{12.705\: \!904\: \!139\: \!414\: \!872\: \!8}}$$
.