solve problem k squared minus 11
\(k^2-11\ \ \ \ |use\ a^2-b^2=(a-b)(a+b)\\\\k^2-(\sqrt{11})^2=(k-\sqrt{11})(k+\sqrt{11})\\\\if\ k^2-11=0\ then\ k-\sqrt{11}=0\ or\ k+\sqrt{11}=0\\\downarrow\\k=\sqrt{11}\ or\ k=-\sqrt{11}\)