Question 146425
You are looking for the perfect square such that, 
{{{(u+a)^2=u^2-12u+d}}}
When you expand the left hand side, you get,
{{{(u+a)^2=u^2+2au+a^2}}}
In your example,
{{{2au=-12u}}}
{{{a=-6}}}
When you match up values, 
{{{d=a^2}}}
{{{d=(-6)^2}}}
{{{d=36}}}