Question 103662
Let's compare it.
{{{(x+a)^2=x^2+highlight(2ax)+a^2}}}Perfect square.
Your equation is 
{{{x^2+highlight(2x)}}}
The highlighted portion gives you
{{{2ax=2x}}}
{{{2ax/2x=2x/2x}}}
{{{a=1}}}
So your {{{a^2}}} term that you need to add to make it a perfect square is 
{{{a^2 = 1}}}