Question 101671
Almost!
{{{H^2=L^2+W^2}}}
{{{12^2=L^2+8^2}}}
{{{144-64=L^2}}}
{{{L^2 = 80}}}
{{{L=sqrt(80)=sqrt(4^2*5)}}}
{{{L=4sqrt(5)}}}
Your process was correct, it was the "simplified" radical that was incomplete.